When I was a child, I walked to school by myself from the age of five. On weekends, I would go outside to play with my friends after breakfast and stay out all day. If my friends and I ran out of things to do and tried to go home, our mothers would be quick to push us out of the door again with sharp lectures about the importance of fresh air. But times have changed. Today, a "responsible" parent would not dream of letting a five year old go to the playground or even ride her bike unless there was an adult watching over her.

Why not? Well, most parents would say it's because it's just not safe. But now, it seems, the new "indoors childhood" is not safe for children either. According to a report in last week's British Journal of Medicine, an "alarming proportion" of British pre-school children are overweight or obese. A study that is following the development of 1,000 children in the Bristol-Avon area has found that by the age of four, 20.3% of them are overweight, while 7.6% are obese.

The figures are slightly lower for five year olds, but the overall figures show a rise in the number of children entering school with weight problems. According to John Reilly, senior lecturer at the department of nutrition at Glasgow university, the expectation before this study was that you'd find about 50 obese children in every 1,000. Now, it seems, you're likely to find about 80. He and his colleagues do not believe that children are getting fatter because they're eating more: other studies have shown that children are actually eating less. If they're gaining weight, it's because they're spending so little time playing outside, and so much time in front of television and computer screens.

This leads on to other problems. Children who get little exercise when they are young will have poorer muscle tone and lower fitness levels. If they spend most of their time inside, they also run the risk of never learning how to play by themselves, or how to learn through play. Organisations such as Families for Freedom are concerned that these constrained conditions will have adverse effects on children's character development - that they will grow up to be fearful, compliant and incapable of independent action. The Mental Health Foundation has also warned that children who never get a chance to play outdoors also never get a chance to learn the basic skills that are essential if they are to be emotionally as well as physically "resilient".

Terrible, isn't it? Everyone is agreed on that much. When the news about fat children hit the public consciousness last week, a number of newscasters went so far as to ask child experts why it had come to this. Their responses were meek and confused, along the lines of: "Well, I suppose parents don't let them outside because they're afraid."

Why parents were afraid was never made quite clear. So let me take this opportunity to fill in a few dots. Let me explain why I don't let my six- and eight-year-old daughters play outside. I should mention first that they are not fat, because I have always made sure they get plenty of exercise. Like so many parents who are uncomfortable about the new indoors culture, I do my best to make up for it by taking them to ballet lessons, gym lessons, swimming lessons and all the rest. Of course it does not make me happy to know that they do not know what it is like to do anything physical without an adult standing over them. But even so, I would never dream of letting them out of my sight. It's just too dangerous.

I live in a village on the edge of Bath that would be a paradise for children if it were not for the car and lorry drivers who think nothing of ripping down country lanes at 40 and 50mph. If they act as though they own the place, it's because they believe they do.

Once, for example, I prevented a man in a sports car from running over a toddler who had tripped when coming out of our village playschool and fallen off the narrow pavement into the road. This driver was beside himself. "It has nothing to do with me," he said. "As far as I'm concerned, that child should not have been there."

I got a variation of this line only this morning, when I had words with the lorry driver from the council library service. He chose to back down the very narrow school lane at 8.55am, when the lane and path were teeming with children. I asked him to stop after he came within millimetres of running over two boys who were riding bikes.

Here are a few of the things he said in response. "But why should I?" "But surely mothers such as your good self will be holding their children's hands and keeping them out of harm's way." "But I can't stay here, I'm blocking traffic." "But I have no choice. We have a delivery to make." "But it's dangerous to stop, too. What would happen if there were a fire?"

He was not a cruel man. He was not even an impolite man. He was just a man who had a job to do. And it was my job, he implied, to make sure that my children stayed well out of his way.

I suspect that most people in this country would agree with him. But in this particular village we've all seen too many near misses, and we're fed up. Now we're trying to get a path built that will allow children to get to school and to the playground beyond without having to negotiate the most dangerous stretch of road. It's all been very slow going - we made our first application two years ago. There's been strong resistance from the local preservation society, which has voiced concern that a path through the valley will upset the badgers.

Lately, they've expressed an interest in a compromise. Even so, the idea that children might be just as important as badgers is still a radical concept. But the most difficult point to get across is what we really want to come out of all this. The aim is not just to cut out car accidents, but to bring back all the things that children can't enjoy these days because of the danger of car accidents. What we want is a village where children can walk to school by themselves and go outside after breakfast to meet their friends - have a life, in other words. Have a proper childhood, the sort that you and I were able to take for granted.

In my view, today's children are getting fatter and more flaccid because we don't give them enough space in which to have a proper childhood. The weight problem is inextricably linked to the space problem, which is inextricably linked to the traffic problem, and it doesn't stop there.

Health, safety, play, child development, the protection of the environment and the regulation of traffic - they might all look like separate issues, but when it comes to children, they are all of a piece. They will not be resolved until they are addressed together.