My twin sons want to play out. Not out in the garden, but out there, in the enormous, terrifying world, without me. 'Lewis plays out,' Sam tells me. 'So does Lucy. And James. And...' He rattles off the names of umpteen kids whose parents are not intent on wrecking their fun.

'They live on a cul-de-sac,' I remind him.

'We could play on their cul-de-sac.'

'I could take you to their cul...'

Sam stomps away. He doesn't want to be taken; that's the whole point. It strikes me that asking to play out is a reasonable request. Like most eight-year-olds, he and his twin brother, Dex, are insatiable climbers, waders and foragers. Why shouldn't he be allowed to roam and explore as I did as a child? Growing up in a west Yorkshire hamlet, I enjoyed limitless freedom from the age of four. Entire summers were spent playing in the woods and a derelict three-storey mill with perilous holes in its rotting floors. No grown-up tagged along to 'supervise'. No one ranted: 'Get out of that river. Do you know how long trainers take to dry? Do you?'

In a decade, only two scary things happened: a man pulled up beside me in his car and asked if I'd like to go to his house to see his puppies (I legged it home) and another man waggled his penis at me from a hillside.

Incidences of child abduction have remained virtually unchanged since 1950, yet today's 'caution culture' has dramatically altered how young people play. The radius around their homes in which they are allowed to roam has shrunk to a ninth of what it was in 1970. Research by the Policy Studies Institute indicates that, while 80 per cent of seven- and eight-year-olds were allowed to walk to school without an adult in 1971, by 1990 that figure had fallen to 9 per cent. 'Traffic is the main factor and it's an absolutely valid concern,' says Adrian Voce, director of the Children's Play Council.

Although the volume of traffic-related accidents and deaths is falling, Britain still has the highest child road-death figures in Europe. In 2003, 186 children were killed on the UK's roads and 4,000 injured. 'Children need adult-free time and the space to enjoy it,' says Voce. 'Unfortunately, there's a feeling they should play only on primary coloured equipment, with an expensive rubber surface beneath and a fence all around.'

My sons are tired of being fenced in. They want to go to the park, a vast area of grassy hills and woodland with a stream running through. The main reason we left London for rural Lanarkshire was to raise our family in a clean, relatively safe environment. I wanted my kids to spend their formative years climbing trees and building dens, not being shepherded from one organised activity to another. But how safe is it here? Sam and Dex are physically adventurous. During one accident-prone period, we were such frequent visitors to A&E that the boys were familiar with every toy in the waiting room. 'Oh, I love this tractor,' they'd enthuse, dripping blood.

For reassurance, I call Rob Wheway, adviser on play safety at the Child Accident Prevention Trust. 'If children are warned about dangers - a busy road or canal, say - they'll tend to keep that in mind,' he says.

'Most kids are more cautious than we imagine. The alternative is to keep them in the house, where there are just as many, if not more, dangers. Children become bored when they're cooped up, and start fiddling - and that's when accidents tend to -happen.' He adds that they are happiest playing fairly close to home, 'knowing that there's a parent or trusted adult to run to if anything untoward -happens'.

With this in mind, I agree to loosen the reins for a week and let my sons leave our garden unaccompanied. They kick off with a play in the park, albeit with a friend's sensible 11-year-old daughter as unofficial minder.

They look vulnerable as they head for the park. These are boys who can't yet tie their shoelaces or pour themselves cereal without splattering the kitchen. The house feels quiet. I have awful visions of head injuries requiring a mercy dash to A&E, although they're probably doing nothing more risky than kicking pebbles into the stream. I consider wandering across on some phoney errand or spying on them, disguised as a bush.

I hover anxiously at the window. My friend's daughter emerges from the alley that leads to the park. Behind her, their clothes sodden, are my sons. One has mud in his ear. Both are shivering. 'We went paddling,' Dex explains, 'and kind of fell in.' They require complete changes of clothes, warm baths and hot chocolate and won't be allowed out unaccompanied until they're in possession of facial hair and driving licences.

However, the following day, the boys beg to go to the newsagents to buy their beloved Dandy. I watch them hare off, figuring that the excursion should take no longer than 15 minutes. They're back in under 10, panting and breathless, in possession not of the aforementioned Dandy but a travel game called Guess Who and packets of Maltesers. They think they left my change on a windowsill.

Flushed with success, Dex demands to go to the Co-op to 'do all your food shopping'. Although I'm attracted to the idea of never again having to roam the aisles with a trolley, I fear that such a task might prove too challenging. My partner, who is making lunch, needs mint and a lemon. I explain to Dex that it's the herb mint that's required here - i.e., green, leafy - and not sweets with a hole in the middle. He scampers off and returns with a jar of dried mint and a bottle of lemon juice ('better than real lemon because there's more juice in it').

As the days pass and my sons run more errands, I begin to feel less fearful. My sons are loving their newfound freedom so much that it's almost hurtful. They are no longer thrilled by making cookies with me or digging up worms in our garden. I'm starting to wonder what possessed us to buy a house with 140 square metres of lawn. 'Beyond the very early years, back gardens are of limited value,' says Wheway. 'Children prefer to play in front of the house where they can see the world going by.'

'Playing in the garden gets boring,' agrees my sons' nine-year-old friend, Isabel. 'It's more fun being out with your friends.'

Her father, George, tells me that when they lived in a quiet Edinburgh suburb, Isabel played out from the age of four. 'Now we live on a main road where our three cats have been killed by traffic, so she has less freedom.'

Given the choice, it seems that kids still prefer running wild to being plugged into a PS2. But what about me? How long before I'm surplus to requirements? If nipping out to the Co-op has become my sons' favourite activity, where will they want to go next? Send a kid out for a lemon and he'll soon be demanding to ride his bike to the next village and going on holiday to Spain with his mates. It's not my kids who aren't ready to be let off the leash - I'm not ready to let them go.

Neighbours have remarked on how grown-up and sensible my boys appear. Wheway says: 'In areas where it's safe to play, children being allowed out on their own increases feelings of community because people keep an eye out for each other's kids.'

Dr Jennifer Cunningham, a Glasgow-based community paediatrician, says: 'In the days when elder siblings were asked to look after younger children, they tended to rise to the responsibility. With no adult intervention, children learn to control their behaviour, to act less impulsively and behave older than their years. In being over-protective, we're keeping them child-like, denying them the chance to make this vital developmental step.'

Where my sons are concerned, the only mishaps have been the stream incident and the dried mint. While walking to school unaccompanied is a step too far, I agree they can come home on their own. It's a 10-minute walk. It's 3.22pm. They'll be leaving school now, probably having forgotten their schoolbags and lunchboxes. 3.25. They'll be halfway home. 3.30. They should be home.

Dex ambles into the porch. 'Where's your brother?' I ask. He shrugs. 'Where's Sam? Did he come home with you? Didn't I tell you to stick together?' He yawns and says he's hungry, wants a biscuit.

I run out to the street. There's no Sam. If anything's happened to him, I'll never forgive myself. I should have been at the school gates, being a good mother, as I have every day for the past three years.

I charge through the house, calling Sam's name, desperately hoping that he's hiding. I check the garden and that's when I see it: a flash of blue - school sweatshirt blue - in the undergrowth at the bottom of our garden. Sam is lying on his belly, leafing idly through the Dandy. 'What?' he says, looking up at me.

'I thought you were lost. Don't ever do that again.'

He frowns at me, as if I've just said something ridiculous, then asks, 'Need anything from the shops?'

· Fiona Gibson's The Fish Finger Years is published by Hodder & Stoughton on 20 June, £10.99.