The guinea pigs are all of a quiver. Maybe the red kites circling overhead have made them nervous. Or perhaps it is just the chill winter air. Then again it might be the two girls who are peering into their hutch. “They are natural,” says one. “Yes,” says her friend, “but they’re a bit big.”

The girls are carrying small pieces of sticky card. Their mission is to scour the school grounds in search of twigs, grass, leaves; anything to attach to their card to make a picture or pattern. Whatever they use has to be natural and, fortunately for the guinea pigs, small. In another corner of the grounds, two boys are crouched on a muddy path, enthusiastically building a tiny “elf house”. The walls are made of twigs and moss supplies a bed. The construction has sparked a conversation about the nasty habits of the little folk. “How do elves chop fairies’ heads off?” asks one.

“Dunno,” says his mate, prodding the soil with a stick. “Do you think they like paddling pools?”

Two women are on hand to advise this Year 3 class from Watlington primary in Oxfordshire. Fiona Danks and Jo Schofield know a lot about elf houses and what counts as natural. They’re the authors of Nature’s Playground, a new book which would look great on a coffee table, but would prefer to be stuffed in a rucksack and taken on a hike. It’s an inspiring invitation to open the double-glazed doors and push the children outside. Injected with its ideas for games and activities, teachers and parents should be immune to that depressing whine, unleashed after 10 minutes in the open air: “Can we go inside now? Pleeeese?”

Fiona, an environmental educationist, and Jo, a professional photographer, worry about the safe, centrally heated lives of many young people. School trips are problematic. Sats squeeze the outdoors out of timetables. Parents complain about PlayStations, but are paranoid about paedophiles and bothered by dirt. “It’s brilliant fun to get really, really mucky,” says Jo. “But it seems it’s acceptable to get dirty playing rugby and football, but not just playing outside, being free.”

They wanted their own children, all five of whom feature in the book, to grow up with muddy memories of ditches, damp leaves, and favourite trees.

They wanted their imaginations sparked by secret dens and hand-made bows and arrows. They encouraged them to smell elderflower, make pictures with berries, and admire the frail beauty of a night-time moth. And they wanted it all to be fun. “There are so many ways in which you can enjoy and explore the environment,” says Fiona, “whether it’s having adventures and climbing trees, or finding out about natural history through pond dipping, or being creative, or using your senses through touching and smelling.

Hopefully the book will appeal to a range of children, whatever their interests.”

And whatever the time of year and wherever they are. The book is divided into seasons, with suggested activities for each. Fiona and Jo stress that nature’s playgrounds aren’t just in the national parks. The Oxfordshire countryside was their inspiration (Fiona works for the Trust for Oxfordshire’s Environment), but corners of gardens or school playgrounds, school fields or inner-city parks will all do. And they’ll do in the rain just as much as in the sun. “We want to help children realise they are part of the environment too,” she says. “Many don’t know the seasons; the only thing that is different in winter is that they wear a coat on their way home to watch telly. If we give them memories of fun and enjoyment outside, in future they’ll want to protect nature.”

Many of the book’s ideas come from Fiona and Jo’s own children, who all went to Watlington primary. They are simple, practical activities to tickle the senses and get the creative sap rising. Lots will work in a school playground. Native American “dream catchers” woven from twigs and grass can be hung on a climbing frame. Ice mobiles can dangle outside the classroom window. Clouds can be watched by lying down anywhere outside. Flags fly as well in an urban playground as they do in the Chilterns. Budding naturalists can try making a nest or identifying pigeon tracks in the mud.

And you don’t need planning permission to build an elf’s house.

Working in miniature - on an elf’s house or a small piece of card - encourages children to look closely, says Fiona. They find all sorts of things and start to learn. “Sticky cards are always popular,” she says, “even with young teenagers. We wanted to make the activities accessible.

You don’t have to do a lot of work, spend a lot of money, or know a lot of stuff. You just need the ideas.” Some of the ideas are more ambitious, better suited to a walk or a school trip than a poke around the playground.

You may not fancy covering your bare chest in black slugs like one little boy featured in the book, but if you do feel like it, just remember to put the slugs back where you found them.

There are ideas for camouflage games and scavenger hunts. And how about going out at night? Fiona and Jo’s sons, Jake and Edward, have made a tradition of celebrating their December birthdays with a party after dark in some local woods. “Boys need to get out,” says Fiona, “and after dark they behave better because it’s just that little bit scary.”

The book is full of tips on how to protect the environment and the children, but the underlying message is that children need adventure and to feel the occasional frisson of fear, even if they have only temporarily lost a woodland trail laid by an adult (who is keeping an eye on them from behind a nearby tree). “Life is full of risk, so the best way to prepare children for life is to ensure that they understand risk for themselves.

Children taught how to whittle and carve with a penknife are less likely to cut themselves than those who haven’t a clue,” says Fiona.

The pair are not alone in wanting youngsters to experience nature. In a survey for Playday 2005, organised by the Children’s Society and the Children’s Play Council, 40 per cent of the 671 children interviewed said they wanted to get outside more. A fifth said they only spent an hour a week playing outdoors. Their complaints are being heeded. The Association of Teachers and Lecturers and a group of charities, including Learning through Landscapes, have just launched a consultation to back up their demand for local authorities to be set minimum play standards for children.

They want government guidance on creating rewarding, play-friendly school grounds and insist that no new school should be built without them.

The Government appears to be listening. In January, consultation closed on the education select committee’s Outdoor Education Manifesto, with results expected in the spring. The manifesto recognises that “high quality education outside the classroom can stimulate and inspire, foster independence and aid personal and social development”. And next month the Big Lottery Fund will tell local authorities how to bid for a slice of pound;175 million that has been allotted for play facilities. Since the cash was announced last year, councils have been appointing “play champions” to co-ordinate their plans (see TES, “Playing safe is just boring”, February 17).

Back at Watlington primary, after sticky cards, the pupils try making a magic carpet. Twigs are laid in a snake shape on the grass and the body is divided into 15 partitions, one for each pair of children. They fill the sections with moss, berries, teasels, ivy, poppy-seed heads - all gathered from Jo and Fiona’s gardens. Two boys are instantly gripped. They decide their catkins have to be laid in straight lines. “It’s just got to be symmetrical, you see.” Another boy heaps a huge pile of everything in his section. “It’s a winter feature,” he says proudly. “A tornado of nature,”

mutters a girl behind him. They’re thrilled by their hour in nature’s playground, organised as part of the school’s art week. “It’s been an extra playtime,” says one. Then they rush off, small human tornadoes keen to finish their elf houses.

Nature’s Playground by Fiona Danks and Jo Schofield is published on March 16 by Frances Lincoln pound;16.99. www.franceslincoln.com. Playday: www.playday.org.uk