Children from Child of Our Time, the BBC One series that follows the development of 25 children born in the early days of this century, are used to being asked questions, but this year was different. This year we were talking about an adult obsession - the desire for a perfect body. Until recently children have been relatively immune to body image issues, but as skinny Bratz dolls become a bestseller and models get ever skinnier, are they now feeling pressurised to be thin? And do they fear fatness?

The boys among our group aged around 8 were, in general, notably less concerned about their bodies when questioned about weight. One sweet-natured boy replied proudly: “I want to be really fat so I can bash people!”, while another admitted, bemused: “I don't know.”

The girls, though, told a different story. One lovely girl whose weight was normal told us: “I don't like my body shape because I think I look really fat”, and a lot of girls wanted to look like the most underweight child on the chart. The desire to be thin starts even earlier, with many six-year-olds wanting to be thinner and most five-year-olds able to talk knowledgeably about dieting. As one mother told me: “Nowadays they're absorbing beliefs about dieting with their mother's milk.”

Our candid Child of Our Time group didn't mince words about what they felt about fatter children. Pointing to the picture of an obese child, we were told “because she is fat she will be nasty” and that “nobody really likes fat people”. This isn't the first time that these opinions have been aired by young children. I witnessed an experiment in which a class of 30 ten-year-olds were asked to comment on pictures of a thin, medium and fat child. Almost without exception the children said that the fat child would be the nastiest and loneliest, while the thin one was the nicest, surrounded by friends.

It's hardly surprising that children are frightened of being fat if friendlessness is the corollary. Children who are overweight are in turn much more likely to be bullied, such as one boy participating in the experiment. “I have been called a fat b****** and asked what bra size I take. So I run away and stay in my room for hours; I just don't like talking to anyone.” So what did he say when he did the test? He said exactly the same as the others, that the fat child was the laziest, most stupid, most selfish and, tellingly, the most scared. This boy loved animals, was immensely gentle with his father's songbirds, and in another world he would have had friends galore.

Internalising the ideal of thinness doesn't make children go on a diet, it makes them anxious. A dip in their fragile self-esteem is facilitated by the mass media, exhorting us and celebrities to be thin at every turn. Dr Helga Dittmar, of Sussex University, found that skinny dolls can increase children's craving for their own skinnier body. Young children tend to absorb what they are told as the absolute truth. If those messages - such as having a perfect body - are impossible to achieve, they can end up feeling guilty and useless.

Anxiety and depression are real problems of our time. Last year, UNICEF reported on the wellbeing of children in 21 of the richest nations in the world and the UK's children came out right at the bottom. Our forthcoming programme, Child of Our Time, The Age of Stress reveals a stress epidemic among children for whom body shape is one of a list that includes exam nerves, constant testing and perfectionism. The number of prescriptions doled out to children under 16 for mental health issues has quadrupled in a decade.

There may be another reason why many people demonise fatness. Obesity is very visible and once we see it people may subconsciously make a judgment. In times gone by, being well covered was a symbol of affluence but society has changed and we now link it with low economic groups who have less chance to buy fresh food and tend to be more overweight.

While too many parents find it hard to feed their children healthy food, the children can be one step ahead. Teachers nowadays make sure that their pupils know about food and how it can help them be well and strong, but children can't always use this information.

There is a new development, however, in our perception of fatness. The Conservative MP Ann Widdecombe apparently looked at a playground full of children and couldn't see one who was overweight. She isn't the only one. Children have got fatter - a quarter of 11 to 15-year-olds and a seventh of 2 to 11-year-olds are obese - but many people don't notice because almost everyone has got fatter.

A friend of mine now in her forties has, unusually, kept the same, normal weight for the past 20 years. Her dress size, though, has not. She has gone down from a 12 to a 10, and now sometimes fits an 8. She hasn't changed, but our notion of what is thin has. This dramatic change means that, without even noticing it, we have become paradoxically rather less concerned about weight than we used to be.

There is no doubt that nowadays a certain amount of fatness is far from being devalued, it is normal. But very obese children are still being bullied and demonised and, at the other end of the spectrum, perfectly ordinary children want to be thinner. Can the media change its spots? Well, maybe it could.

Dittmar found that “contrary to claims that thinness sells, the effectiveness of the adverts was not influenced by the body size of the models. Average-sized and ultra-thin models were equally effective.” If Dittmar is right and size zero is really becoming less attractive, we should hope to see a resurgence in healthily built role models at last.

Dr Tessa Livingstone is executive producer of Child of Our Time and author of Child of Our Time: Early Learning, published by Transworld. Child of Our Time, The Age of Stress is broadcast on BBC One on Wednesday, 8pm

Times recommends

• Live chat: Thrifty ways to update your winter wardrobe
• Is anybody out there really listening?
• Nutrition Q&A: Will eating late make me fat?