Suddenly it is spring and the garden is green and twitchy; adventure is in the air and my six- year-old and his friend Rosie have found a hole in the fence. Not just any old hole, oh no (sadly, our fences are full of them), but a thrilling and wondrous hole. It's a trouser-snagging gap through which only a six-year-old could squeeze, opening on to a patch of waste ground. The Junk Yard, Jacob and Rosie have dubbed it.

"He can't go there," I wail when it turns out this was where he was when he slipped out of sight for four long minutes. "We can't see him. Anyone might be there."

"Oh, come on," his father soberly argues. "It's what six-year-olds do."

Well, not mine.

No one's ever been sure who owns this piece of waste ground, though people have occasionally been seen hacking at its weeds. It's delectably tangled and prickly. It has a pets' grave on it. Jacob assures me that every surrounding garden can be networked from it.

"It's absolutely not safe," I tell the assembled, scowling men of the family. "I understand why he wants to go there and I'm not cross, but I can't believe it's a good idea." Then I go upstairs and sit on the bed and wonder why I sound like such a bore, wonder what exactly has changed since I personally squeezed on hands and knees through gaps in the fence.

Because openings on to new worlds are what childhood is all about. Where would we be without the wardrobe door that reveals the lamp-lit snows of Narnia, the askew world on the other side of the Looking Glass, the rusty door to the Secret Garden? Peter Rabbit absolutely has to slip under that gate into Mr McGregor's garden, and no amount of punishing camomile tea is likely to put him off. And Kirrin Island, with its dungeons and ruined castle and not an adult in sight, is an adventure just beggingto happen.

Parents - and especially mothers - have to come to terms with the adventures their children will have. When Jacob was less then 24 hours old, asleep on a fleece on the vast landscape of our bed, I stood and looked at him and burst into tears. Because I knew that one day he would get on a motorbike and I would not be able to bear it.

These days, of course, I know better. I pull myself together. I don't want my children to be 39, wearing cardigans and living at home. So when they're at the top of the climbing frame, holding on with one hand and communing with the sparrows, I take

several deep breaths, unclench my fists and relax.

When I was eight or so, we lived in the country and my sisters and I ran riot. We dressed as gypsies with curtain rings for earrings and roamed the fields with just a compass, praying we'd get lost.

We rubbed flowerbed on our faces, knocked on doors in our rags and asked for drinks and peddled lavender and windfallen apples. We were met with everything betweengood humour and plain hostility. We were imitating Charley, the romantic child-beggar in a (deeply enthralling) book we'd read. We wanted desperately to live rough - Charley slept in a chicken hut and ate berries and stole a bottle of milk - but made sure we were home by 6.30 to watch The Champions on television.

Later, we started a Ghost Club whose brief was to investigate "suspicious things". We peered into parked cars and were chased by cows and shouted at by farmers, but we were unabashed. Once a man with an artificial eye swore and threw a potato at us - a terrific detail which we noted later in the Club Log.

All of this was done with full parental assent and full (well, more or less) parental knowledge. Am I now just filled with uptight, urban angst? If we lived in the country instead of south London, would I be happy for my children to roam free and knock on doors? I doubt it. Once my kids are old enough to go on motorbikes, the matter will, of course, be out of my hands. That's what I was weeping for six years ago.

There are dangers we can take sensible precautions to avoid (believe me, I've done the Red Cross resuscitation and choking courses, I have the text books and the fact sheets), and there are those we cannot.

Whatever we do, life will keep on happening to us, and a piece of waste ground is just a piece of waste ground and - if you think about it calmly - only just out of sight of the kitchen window. And Jacob's friend Rosie is, after all, nearly eight and infinitely sensible.

It's a bright April evening and I'm coming to terms with all of this, when my sister (she of the above-mentioned Ghost Club) is mugged, on her birthday, a few hundred yards from our house. She is shocked, grazed and, because she instinctively tried to hang on to her bag (a birthday present that morning), her finger is broken in several places and will need surgery.

The police crowd our sitting-room with their radio crackle and navy blue. The kids, bathed and pyjama-ed and ready to give their auntie her birthday presents, hang around, abashed.

The police say they'll take us to casualty, and I go to fetch my shoes from the garden. It is a perfect, sunny, spring evening. The World Beyond the Fence - delicious, dappled with light and shade - beckons.