There was a time when most children spent their idle days like this. Ollie and Woody are clambering up the inside of the tree they call their rocket. They bicker over who is going to get to ride in the cradle of the trunk. There's room for two, but Ollie wants to go it alone. The scene is a timeless one. Rewind 20 years and it's possible that one might have found two small children playing in the same spot of a Fife forest. But it is also, perhaps, a rare one for our times: all the more so because of the weather. It is cold, wet, blustery. Ollie, dressed in waterproof overalls, moans that he doesn't like the rain and wants to go back to the house. It's a complaint that belongs to a long lost era when children were sent outside whether they liked it or not.

These two-year-olds, of course, are not out in the woods alone. They are accompanied by their childminder, Cathy Bache, creator of The Secret Garden, an outdoor nursery, whose days revolve around mud, rain, hot lemon drinks and woodland wildlife. Though there are other nurseries in the UK that incorporate extensive outdoor time, hers is the only one that commits to it full-time. Bache believes that a daily dose of the elements is good for mental health. While the younger children spend only half-days outside, those over three years old are out for seven hours, rain or shine, blizzard or heatwave, and only permitted into the house to go to the bathroom. Mostly, she says, her 20 children do not complain about the cold. If their kit is appropriate, they don't seem to notice. If their hands chill, she warms them under her jumper. Bache doesn't check the weather forecast the day before. "We're going to be out in it regardless."

This approach was partly inspired by her time in Norway, where she raised her first two children. It is a very Scandinavian concept: the first wood kindergartens were set up in Denmark in the 1950s."In Norway," she says, "you just go out in bad weather. It's almost a mistreatment if your child doesn't get out for one full hour every day, regardless of weather."

As we enter the woods, my own eight-month-old baby is strapped to my back, dressed in a borrowed waterproof suit, so big that its arms form flippers in the wind. The rain lashes down, wrinkling bared fingers. By most accounts, this is miserable weather. The two boys pause at the bottom of a path where the water floods down in a small stream. They scoop up water in large plastic cups and offer it to us to drink. Rainy days, Bache says, are far more interesting than dry, warm, sunny days. There is more to do. "We make dams, we make mud pies."

Bache likes to take a back seat. It's as if she is there more to facilitate than instruct. The children choose their own routes; invent their own games. In one area of the wood, she tells me, they are pirates, in another tigers. She suggests that we wander to the shelter where we can have a picnic snack. It will be dry in there. Ollie, who took against the rain before, now wants to stay out and informs us that he likes the wet.

Bache's approach is one she believes might improve mental health regardless of age. About seven years ago, she started to feel she had found a way of dealing with the depression that afflicted her and ran in her family. She had started to take a daily walk in the local Fife countryside, taking with her a book of one-line nature poems by Thomas A Clark. The walking and contact with nature helped her to deal with the "black dog". "I engage with the weather with my moods and it shifts them. The way I've coped with my depression has been to engage with the outdoors rather than sitting inside and being miserable."

One of her latest ideas is rain therapy. "Bad weather classes" she believes, might help the "national health bill go down". In the UK, she says, "everybody gets down when the skies turn grey, but we get so much bad weather we've got to get through it. You do that by going out in it. If everything is getting on top of me the best weather to go out in is pinging rain. It just takes it all away."

No demographic group, she believes, is more divorced from the elements than young children. "It's those born in the past 15 years who really have missed out on the outdoor experience, partly because of the PlayStation and television. But what are the experiences most people remember when they talk about their favourite childhood days? They're almost always outside."

Bache believes early experience with nature is formative. "I think with the kids I am caring for, even if they do end up as normal teenagers indoors with their PlayStations, there is a little seed planted. Later on, when they've got their own free will, hopefully they will develop that in their own way."

It is her own memories of childhood times with her grandfather, Tom Bartlett, that she recalls as she takes her walks. Bartlett, who died when she was 12, would take her and many other local children up into the woods to play. There, he built fires and told stories about Peter Pan and Hiawatha. "I think he was a bit of a Peter Pan himself: never grew up and when he died, in his 80s, was still having bonfires."

Bartlett was an ornithologist, who, in the late 1940s and 1950s, helped created a city nature reserve in London. "Memories of my time with him came back to me when I returned from Norway," she recalls, "I got stuck inside with the kids and I was trying to find ways of getting out. Then I remembered all this stuff my grandad did with me."

Ollie and Woody move at a snail's pace through the woods. It can, Bache tells me, sometimes take hours just to get up the path to the edge of the trees. They meander here and there, guided by whim. This isn't the kind of walking that has become a modern, adult leisure pursuit, the bagging of Munros, the strict tracing of a well-worn and mapped path. This is wandering. Bache, herself, walks like this. One of her favourite Thomas A Clark quotes is: "There are walks on which we tread in the footsteps of others, walks on which we strike out entirely for ourselves."

Once, on one of her walks, Bache left home and kept going for 26 miles. She daily gets up early to meditate for half an hour before stepping out of her door, even in the dark of winter. "The dark is fine, just different. There's a lovely silence that you don't get at other times."

Bache treads a considered line between risk and safety. Her children learn to light fires and saw wood, though only under supervision. She recommends a book by Canadian Michael Ungar, Too Safe For Their Own Good. In it Ungar describes how "in our mania to provide emotional lifejackets around our kids, helmets and seatbelts, an endless stream of evening programming, and no place to hang out but the tiled flooring of our local mall, we parents are accidentally creating a generation of youth who are not ready for life. Our children are too safe for their own good."

The Secret Garden, though only a childminding business now, opens as a proper nursery in September, when it gets its own premises in a village hall. That building, of course, will be checked against all health and safety regulations, though it will remain empty. Meanwhile, who will be assessing the local trees, leaves and streams? As yet, luckily, nobody.

In some senses Bache is preaching to the converted. The children that come to her are relative woodland veterans compared with city kids. Ollie's mother Paula Cowle says: "I do have him in the outdoors quite a lot anyway." Bache acknowledges this and has in mind to set up an outdoor nursery in London. There, she has already seen the sort of site she craves: an old cemetery. "A third of it is graves and the rest is just wilderness, the ideal outdoor nursery."