The new child obesity strategy that was launched last month illuminated the challenge faced by policy makers — and those delivering policy — in seeking to change "intimate" behaviour. And few things are more intimate than stuffing your face.

Failure to stop the rise in the numbers of children who are overweight will incur a heavy price. If trends continue, a quarter of children and the majority of the adult population will be obese by 2050, according to a report by the government's Foresight committee. It calculated the consequences – soaring diabetes, strokes and heart disease – would cost the NHS £6.5bn alone, while additional impacts, such as a high proportion of the population unable to work, would take the total bill to £49bn.

Turning this situation around requires drastic changes in behaviour and a level of joining up that the public sector finds extremely difficult. It is not simply a question of getting children, and their parents, to eat less. A recent report from the prime minister's strategy unit noted that compared to the late 1970s, 7-12 year-olds are consuming fewer calories, not more. Yet according to data from the health survey for England, child obesity among the under 11s in 2005 rose to 17%, compared to 2002-04 when it was 14.9% – a 70% increase on 1995 figures.

The inexorable rise in obesity can be attributed to modern lifestyles: TV, computer games and other technology makes it easier to get entertainment at home, while safety concerns mean that children are less likely to walk or cycle to school. Insufficient secure and well-equipped play areas and the sale of school playing fields have made it harder to be physically active. As a result, children are not burning off the calories they have consumed, causing them to put on weight.

Previous attempts to deal with child obesity have largely foundered. In 2004, the departments for culture, media and sport; education and skills (now children, schools and families); and health agreed a joint public service agreement "to halt by 2010 the year on year increase in obesity among children under 11" as part of a wider obesity strategy.

Dame Sue Street, the former permanent secretary at the DCMS, now a strategic advisor to Deloitte, says it was incredibly difficult to work across government - "not because departments were not willing to work together, but because the goal had different priority in different departments and there were conflicting messages from front line professionals. We were not always sure what was the right thing to do. There isn't a manual for tackling child obesity."

The three departments jointly funded a single manager for the public service agreement and established a programme board to provide "strategic direction" and oversee action. More than 20 initiatives have been introduced to tackle child obesity. Weight watchers

These include a national programme to weigh four to five and 10-11 year-olds; measures to improve nutrition in schools; the so-called "healthy schools programme" which seeks to promote healthy eating, emotional health and wellbeing as well as sport; traffic light food labelling; the five-aday fruit and vegetable campaign; making routes to school safer to promote walking/cycling; revamping playgrounds; and restrictions on junk food advertising.

The number alone suggests a scattergun approach to tackling the problem. Tam Fry, chair of the Child Growth Foundation and a board member of the National Obesity Forum, says that until the latest strategy, the government only scored two out of 10 for a coordinated child obesity strategy. "The parent has the prime responsibility of raising the child but only the government can ensure there are enough playing fields and regulate supermarkets so that fast food is healthier," he says.

A joint assessment by the Audit Commission, National Audit Office and the Healthcare Commission in 2006 concluded that while a multifaceted approach to child obesity is the most effective, "children most at risk may be reluctant to participate in such programmes without individual support and encouragement."

While some areas, such as Bradford and Bolton (see below) seem to be bucking the trend, there is little evidence so far that the latest child obesity strategy or the joint PSA on children's health for the 2008-11 spending period will improve coordination. In fact, the new PSA seems to be going backwards – instead of halting the rise in obesity by 2010, it promises to reduce it by 2020. While the government claims the new target is more ambitious in that it aims to reduce the numbers of overweight children, others are more sceptical, pointing out that reducing obesity and overweight to 2000 levels is a damning indictment of hundreds of thousands of children.

But the government is bullish about its new strategy – it aims to become the world leader in the fight against child obesity. Progress will be assessed and published annually. One priority is prevention through improving nutrition during pregnancy and early years.

Local authorities will be given powers to prevent fast food outlets opening near parks and schools. More action is promised to make food healthier, through reducing sugar, salt and additives in food. The government is starting to make headway in tackling school nutrition, although much of the credit for improving school meals goes to Jamie Oliver.

The children's department has already promised to extend its healthy school meals programme – it will invest £500m between 2005-2011. Secondary school pupils will have compulsory cooking lessons from 2011. But getting children to eat healthily is still an uphill struggle. Even though the five a day campaign has been running since 2003, only 17% of children actually eat at least five portions a day.

The culture and children's departments want all children doing five hours of school sport a week by 2010 while a new play strategy will make it easier to encourage children who don't like sport to be active. A healthy community challenge fund is planned for councils to promote physical activity. How many of the five hours will actually consist of activity remains to be seen – although 86% of children have two hours of physical education a week, ahead of the 2008 target of 85%, pupils are not always getting the full two hours of physical education because this includes the time to transfer to the sporting venue and get changed.

"Left to myself I'd focus on more imaginative ways of helping children who feel threatened by school sport. We have to find ways of motivating children to get off the sofa without pushing them all into competitive sport. We have to make healthy eating cool," says Street. And the new strategy plans to work with the computer games industry so parents can limit the time children spend online.

But these initiatives risk being undermined by junk food advertising. Adverts for foods high in fat, salt and sugar are banned during programmes aimed at children under 15. But experts say this does not go far enough, as many children under 15 watch adult programmes that are not covered by the ban. The government has asked Ofcom to bring forward
its review of these regulations to see if they need to be tightened.

There are growing calls for child obesity to be treated as neglect. If a child is malnourished, social workers will immediately hold a case conference, but if he or she is obese, they say it is the parents' problem, even though the health of the child may be at stake.

· Healthy kids
Councils and primary care trusts in Bradford and Bolton are doing innovative work, supported by local sports teams. In Bolton, a joint initiative between Bolton council, Bolton Wanderers and the PCT aims to encourage children to eat healthily and educate families on how to pack a healthy lunch box. Youngsters in 10 primary schools in Bolton have been split into "football teams".

Pupils with a nutritious lunch box or who choose a healthy school dinner will score "goals" for their team. All ten teams will be entered into a prize draw for the chance to dine with the Wanderers. Fifteen playgrounds have been refurbished and 35 new ones are in the pipeline,
while all under 16s can go swimming for free.

In Bradford, the PCT, Yorkshire Sport and the local authority have jointly funded a childhood obesity coach who works in partnership with Bradford Bulls to encourage children to become more active.

· This article appears in the new edition of the Guardian's Public magazine.