Marcus Dolton was born on the small side. He weighed 5lb 6oz. “He wasn’t always hungry as a baby,” his mother, Amanda, recalls, showing me a photograph of him as a blond-haired, blue-eyed, round-cheeked seven-month-old. When Marcus was three he was just “chubby”, Amanda says, showing me another picture, taken on his first day at nursery school dressed in a baggy new uniform. She finds one of Marcus at six. He has noticeably put on weight. Taken at his school sports day, he is wearing kit that’s a little too tight. By the time Marcus was eight, he was wearing 13-year-old’s clothes.Marcus sits in the room next door on the computer. He is 12 years old and 13st. Three years ago Amanda went with Marcus to a paediatrician, who told her “because of his weight it was neglect”. Then social services became involved.

In my eyes, neglect is when you’re not feeding your child. When you’re not listening to them or buying them clothes

The World Health Organisation considers obesity one of the most serious public-health challenges of the 21st century. Obesity exposes individuals to a greater risk of heart disease, asthma, type-2 diabetes, cancer and strokes. It costs the NHS an estimated £4.2bn a year. In the UK, according to the latest figures, a third of 10- to 11-year-olds and more than a fifth of four- to five-year-olds are overweight or obese. By 2050, obesity is predicted to affect 25% of children.

So who, or what, is responsible for children’s weight issues? And what should be done?

In 2006, Sir Liam Donaldson, then chief medical officer, warned that social services may consider removing children with weight issues from their families. The first reported case came the following year: an eight-year-old girl from Cumbria was taken into care weighing 10st. There are no official figures on how many children have since been taken into care because of their weight. I sent freedom of information requests to 156 councils across the UK asking how many times, in the past three years, social services had taken a child into care or “intervened in some shape or form” because of the child’s weight. The majority of councils rejected the request, claiming it would take too long to establish. But eight councils identified up to 31 cases where they had intervened. In Redcar, one child placed on a care order to receive compulsory services at home was 19½ stone.

Amanda, a slim woman, sits in her kitchen in Selby cutting out and taping cardboard buses for her second son, Trey, who is 11. Four other children play somewhere in the house, including Marcus, who does not want to be involved in our chat. It is a tricky subject. Amanda whispers as she tells me how hurtful it was when social services described her son’s weight as neglect: “It makes you feel like it’s your own fault that your child is overweight, that it’s your failure as a parent.”

She points out that “neglect” is the word she heard used recently to describe an appalling case where a child was battered and killed. “In my eyes, neglect is when you’re not feeding the child, when you’re not listening to the child, when you’re not buying them clothes.” Is it neglect when a child is overfed?

On the shelf, under the window in Amanda’s kitchen window, is a row of cookery books. It includes Mary Berry’s Kitchen Favourites and another called Take One Veg. “We tried to be vegan,” Amanda explains.“With Marcus, we’ve tried everything. We’ve tried couscous …”

“Urgh,” interjects Trey, playing at our feet.

“… We’ve tried gluten-free bread, dairy-free ice cream — I wondered if cutting down his gluten and dairy intake would help.” Marcus drinks Diet Coke and bottled water. In the cupboard are “natural” sweets, a Weight Watchers tuna meal, sugar-free jam. “If you want things like that you’ve got to spend money,” Amanda says. “I am trying.”

She shows me pictures of a packed lunch she made him. In it is a salad shaped like a clown: a piece of cheese as the body, grated carrots for hair and a tomato for a head. Amanda looks up health advice on the internet, but the information is conflicting.

“Some people say you should have a low-fat diet, some people say you should eat high-fat.” To make things even more difficult, Trey is autistic and doesn’t like trying new food. Whatever she has tried, Marcus has carried on putting on weight.

Amanda discovered Marcus was sneaking to the kitchen between meals. He would fill a cup with cereal and creep to his room. “The worst is when you decide to go to the toilet — everything gets quiet and that’s when they’re in the kitchen,” Amanda sighs. She has started locking the fridge.

At school, it is impossible to know what Marcus is eating. “He can tell me he’s having a sandwich, but I don’t really know.” She’s stopped his pocket money, so he can’t buy sweets on the way home from school, but she can’t stop other children giving him theirs. Nor can she stop him being confronted with pizza, cake and sweets when he goes to birthday parties.

Amanda doesn’t want Marcus to feel like he’s being singled out. “He sees it as being punished because he’s overweight.” She admits she sometimes gives in and buys him sweets. For his birthday, they went to McDonald’s. She knows Marcus could do more exercise, but he is self-conscious in PE. She has tried getting him boxing lessons, an Xbox Kinect (a gaming console that encourages physical movement) and buying a cross trainer. “The other children were going on it,” she says.

Amanda thinks that Marcus comfort eats whenever Trey has a “meltdown” caused by his autism, during which Trey screams and hits himself. “He doesn’t like the noise and I’m not there for him because I’ve got to be there for Trey.” Although she has tried to explain the health implications of obesity to Marcus, Amanda doesn’t feel that a 12-year-old can fully understand them.

In Brighton, Jeanette Cowan tells a similar story about her daughter, Samantha Packham. “When she was 14, I told Sam that her weight could kill her. She looked right at me and said, ‘Mum, don’t exaggerate.’ ”

When Sam was born, she weighed only 5lb 4oz. “A very small baby,” Jeanette says fondly. But she had feeding issues. Unable to keep her bottles down, she lost so much weight that she was admitted to hospital. Doctors diagnosed a problem with her stomach lining and switched her to soya milk. At first, it was a blessing: Sam was putting on weight. “Until she got to about two,” says Jeanette. That was when Sam’s eating changed. “She was quite big. People would mistake her for a six-year-old.”

Jeanette and Malcolm, Sam’s dad, discussed her weight with doctors, who reassured them “it was puppy fat” and that “as Sam got more active, she’d lose it”. She did not lose it. By the time Sam was eight, Jeanette realised her weight was becoming an issue. “She was happy … but she wanted food all the time.”

Tragic affliction: Samantha Packham with her parents, Jeanette and Malcolm. She died aged 20 in July 2015

Occasionally, Jeanette tried putting Sam on diets, but “when someone wants food, they will find a way of getting it. And Sam did.” A secret eater, Sam would sneak downstairs at night to eat; she would tell other parents she hadn’t been fed. At school, other kids gave her food. By the time she was 14 years old, Brighton and Hove city council received an anonymous report of neglect.

Initially, social services found a nutritionist, but although Jeanette says the family tried healthy eating, Sam continued to put on weight. She was offered a gastric band. “I was shocked,” says Jeanette. “I thought, ‘She’s 14 years old, it’s so drastic.’ ” Jeanette says that at the appointment Sam was “petrified”, crying hysterically and begging to go home. Sam and Jeanette refused the operation, and social services subsequently took Sam into care.

Initially placed in a specialist children’s unit, Sam lost 1½st in six months. But later she was moved to a foster home, where, according to her mother, Sam had a strained relationship with the family. She ran away five times.

Samantha as a tween

When Sam returned home, just before her 18th birthday, she was far larger than when she had gone into care. By 20, she weighed 40st. Unable to walk, she was dependent on her parents and chronically lonely. “She wouldn’t meet anybody, she would only go out if she had to, and if she talked to anyone it was through the internet.” One night, Sam called Jeanette in pain. A doctor diagnosed an infection and she was admitted to the Royal Sussex County Hospital, where they found her internal organs were inflamed. “She looked me in the eye and asked, ‘Am I going to die?’ ” says Jeanette. “I said, ‘Don’t be silly.’ But you kind of know …” When she died of a heart attack last July, Sam became one of the youngest people in Britain to die from morbid obesity.

“We should have been stricter, we should have done a hell of a lot more,” says Jeanette. Still, she is offended by accusations of abuse. “We always made sure we gave Sam hugs, we kissed her goodnight. Sam never went without. Sam’s last words were, ‘I love you, Mum.’ ”

Jeanette is critical of social services. “They don’t understand obesity,” she says. She believes obesity is both a physical and mental issue. She believes Sam had Prader-Willi syndrome, a genetic condition that causes overeating. She thinks children with obesity issues need counselling. “People assume if someone is fat you can go on a healthy diet, exercise and the weight will come off, but it’s not as simple as that. A lot of it is emotional — it’s low self-esteem. Sam would say she hated the way she looked, she didn’t want to be like this, but she was scared. You’ve got to deal with the root of the problem before you can deal with the rest.”

There are a million tiny battles every day. People who haven’t dealt with it personally don’t know how hard this issue is

Joanna Nicolas, a social worker for 21 years, has personally dealt with at least six child-protection cases where obesity was a factor. She does not mince her words. In her opinion, “child obesity is absolutely neglect. I understand why parents find it hard to hear that, but it is. It is the persistent failure to meet the child’s physical and emotional needs.” She tells me of one child whose weight left him unable to climb the stairs to the second floor of his school. “The head teacher asked me, ‘Is that neglect?’ I said, ‘How could it not be?’ ”

In Nicolas’s experience, obesity is rarely an isolated problem. She recalls a case where one parent was terminally ill and the other was rewarding the child with food: “Food becomes an emotional coping mechanism. You just wouldn’t find a morbidly obese child in the middle of a happy, healthy house,” she says. “Most parents, if a child starts putting on weight, they deal with it.”

When do you put a child on a diet? Would you do it if your two-year-old was overweight? Is seven too early? When is too late? Bea was seven years old when her mother, Dara-Lynn Weiss, decided to do something “fairly severe” about her weight.

Dara-Lynn, a middle-class New York parent, was always hot on healthy eating with her kids. She admits she felt “some disdain for moms who let their kids drink soda or eat fast food” and “sometimes looked down on the moms of fat kids”. There was no junk food in her house. Her kids drank only water and ate three healthy meals. Yet by seven years old, Bea was 4ft 4in and 6½st. “She was well into the obese category,” Dara-Lynn says. She was advised by a doctor to address Bea’s weight.

The family began seeing a dietician and following a traffic-light diet similar to that of Weight Watchers. Putting a child on a diet is not easy. There were “a million tiny little battles every day. There was not a quarter of a day that would go by without an argument. I don’t think people who haven’t dealt with this issue personally know how hard it is to be successful and what it takes. It is unrelenting. And it’s for ever, which is the hardest thing.”

We need to act on obesity in the same way as we act on drugs and alcohol, because we are facing an epidemic

She fought Bea’s constant complaints and managed her daughter’s emotions, as well as her own guilt, when Bea’s skinny younger brother wolfed down food she wasn’t allowed. It felt “super-cruel and stigmatising”. At other children’s birthday parties, Dara-Lynn would deny Bea an extra piece of cake; she battled to check the calorie content of her school lunches and contended with other parents horrified to see Bea eat low-calorie junk food and Diet Coke. “They’d give her juice and a granola bar, or they felt ‘kids should be kids’.” Still, Dara-Lynn persisted with the rigid diet. She compares weight issues to a child with a peanut allergy, epilepsy or ADHD. Bea had “a disease that it was my responsibility to treat. It is very, very hard to address this issue. You have to work hard to get minimal progress. You look at what I went through fighting this fight every day for a year, and Bea lost on average ¼lb a week.”

Dara-Lynn wrote about her experience in an article for Vogue and a book, The Heavy. The backlash was furious. She was described as “reprehensible”, “contemptible” and “revolting”. The US women’s website Jezebel described her as “the most f*****-up, selfish woman to ever grace the magazine’s pages”. Dara-Lynn admits she wrestled her own anxiety that she might be projecting her own body issues onto her daughter. Still, she insists, “it certainly wasn’t about being thin”. The weight they worked towards was “the very borderline between healthy and overweight”. They have achieved this, but the long-term outcome, says Dara-Lynn, “remains to be seen”. Bea is now 13 and seems good-humoured about her experience, though Dara-Lynn says: “I don’t want to say it was great. Sometimes I’ll uncover something she’s said to a friend that surprises me and I realise perhaps there was more frustration or pain involved in this process than she let on.

“Do you do something or not do something? The mum who does or doesn’t is each horrible in her own way. The problem at the core of it, I think, is who is a worse mum? The one who does nothing or the one who does what I did?”

Chips with everything: the lure of junk food, readily available near their school, proves too much for these south London teenagers

In the waiting room of the children’s ward at Medway Maritime Hospital in Gillingham sits a little plastic wendy house. Beside it is a toddler-height plastic kitchen complete with plastic toaster, plastic watermelon and plastic corn on the cob. On the wall are details for a healthy eating programme aimed at children between the ages of two and four. I came here to met Ashish Desai. A consultant paediatric surgeon based at King’s College Hospital in London, he holds two surgical weight-management clinics a month, seeing about six children each time. Social services are involved with “quite a few” of his patients.

Desai’s team takes a holistic approach to weight loss: dieticians, psychologists and nutritionists devise bespoke eating plans, educate patients on healthy eating and recommend fitness apps. With strong family support, many children with early-obesity issues — a body-mass index (BMI) of 30-35 — are able to lose weight, although they may struggle to sustain it. “The problem is, by the time people come to me, their BMI is way above 40,” Desai says. A healthy BMI ranges from 18.5 to 24.9. To be eligible for weight-loss surgery, it must be more than 40. The average BMI for the patients Desai operates on is 50. One 14-year-old he performed surgery on had a BMI of 68.

Surgery is “always a last resort”, Desai says, although he believes that “because there is such strong evidence that this will help save the life of these patients in the long run, I think we should be offering more of this”. Weight-loss surgery can have striking health benefits, such as reducing the risk of heart failure, arthritis and depression. One study found that 84% of patients with type 2 diabetes were completely cured by the procedure.

The World Health Organisation has expressed concern over weight-loss surgery in children, as there has been insufficient research into the impact on their unique metabolic needs and the long-term effects. Yet Desai is adamant. “After the obesity has been established as a problem, then there is no point in restricting access to an operation that is going to be life-saving for these patients.” He tells me about one 14-year-old boy he treated who weighed 30st. After surgery, he lost almost half his body weight. A healthy dieter, especially a child requiring adequate calories and nutrients to grow, should lose a maximum of 4lb a month. At that rate, it would have taken Desai’s patient more than four years to lose 15st.

Desai does not deny that parents can be responsible for their children’s obesity, but he believes there are other significant factors. He notes that obesity is rising most in children aged 5-11. He suggests the reason is that this is when children begin to become more independent: starting school and walking home with friends, in a society that surrounds them with unhealthy options. “The whole of society is going towards TV, apps, computers and mobile phones. People don’t go out to play games any more,” he says, bemoaning fast-food restaurants serving cheap pizzas, burgers and fizzy drinks in promotions aimed at kids. Desai believes the government must do more than the sugar tax. “We need to act on it now in the same way as we act on smoking and alcohol, because we’re facing an epidemic.”

He notes that weight issues often occur in low socioeconomic households. Not just because healthy food is more expensive, but because there are numerous ways that financial struggle can affect families’ lifestyle and diet. He tells me about one mother who works night shifts, and who leaves snack foods out to comfort her child in case he gets up at night while she is out.

Desai tells me that some children develop weight issues following other illnesses. One 13-year-old boy he operated on had grown up with brittle-bone disease, and his well-meaning grandmother had been feeding him up. Another girl who had recovered from cancer as an eight-year-old had parents comforting her with food. Desai tells me that children born prematurely or underweight have a higher chance of becoming obese because parents overfeed them while helping them “catch up”. “By the time they realise it’s a problem, it is too late.”

Fewer than 10% of Desai’s patients are even considered for surgery, and less than half of those receive it. That adds up to three or four patients a year. This is because the National Institute for Health and Care Excellence recommends surgery for children only in “exceptional circumstances” and only once a child has reached puberty — typically around 13 for girls and 15 for boys. In Saudi Arabia, where medical practitioners are less stringently regulated, surgery has been performed on much younger children: in 2013, a two-year-old who weighed 5st received a gastric bypass. “The stigma attached to surgery needs to go,” Desai says. “We’re putting our heads in the sand by saying you shouldn’t offer them surgery. It is the only sustainable route to weight loss.”

Back in Selby, social services closed Amanda’s case after two years. She is still trying diets for Marcus. I ask if she would consider surgery. “I don’t know,” she says. “On a child? Isn’t it abuse?” The last time we spoke, she was enthusing about a nut-free peanut butter she had found. I left her house, turned right towards the train station and saw a McDonald’s at the end of the road.