When you have designed a woodland playground, it clearly helps if there are children to test it - so naturally the Duchess of Cambridge turned to her own little scamps for help.

Prince George, Princess Charlotte and Prince Louis had a whale of a time during a sneak peek of Kate's stunning garden at the Royal Horticultural Society's Chelsea Flower Show.

The Duchess of Cambridge will unveil her woodland wilderness garden - which aims to encourage children to spend more time outdoors - at the event in London this week.

Prince Louis stole the show by making his first steps on camera. Photo / AP.

A barefoot George and Charlotte were seen dangling their legs over a stream, while an excited Louis ran along a wooden path carrying a branch as the Duke of Cambridge looked on smiling.

Four-year-old Charlotte, dressed in a floral frock, and one-year-old Louis, wearing shorts and a cardigan, could be seen trying out a swing seat which Kate has already been photographed on in the build-up to this week's show.

Five-year-old George could be seen adventurously clambering across the stream.

In new photos released by the Duke and Duchess of Cambridge, the couple's three children play in the Back to Nature Garden. Photo / AP.

George, five, Charlotte, four, and Louis, one, would have been in familiar territory. For not only did Kate use their time playing outdoors as inspiration for the project, which will open to the public on Tuesday, but the children were actively involved in collecting twigs, logs and moss from around Anmer Hall, their Norfolk home.

They built a rustic tepee-style den and fire pit at the heart of the garden.

'They've been involved every step of the way and the duchess was excited for them to see the finished result,' said an aide. 'Children – both hers and others – have always been at the heart of this project.'

The Palace confirms George, Charlotte, and Louis gathered sticks, leaves and the pine cones from their home at Anmer Hall for the garden. Photo / AP.

Pictures taken by portrait photographer Matt Porteous show Charlotte, barefoot in a pretty pink summer dress, on a rope-ball swing and Louis under the watchful eye of his father Prince William. George and Charlotte are also seen dipping their toes in a babbling brook.

The space, which Kate hopes will inspire families 'to get outside' and 'enjoy nature', features a tree house, waterfall, rustic den and a campfire as well as tree stumps, stepping stones and a hollow log for children to play on.

George, Charlotte and Louis contributed to their mother's creation by collecting leaves, moss and twigs over the past months which were then incorporated into the surroundings of the campfire.

Hazel sticks collected by the family were also used to make the garden's den.

In the pictures, taken by photographer Porteous and released by Kensington Palace, the family could be seen sitting next to the campfire and den.

Toddler Louis looked fascinated as he held a stone, while Kate, kneeling at his side, wrapped her arm around her youngest child.

The RHS Back to Nature Garden, designed by Kate with landscape architects Andree Davies and Adam White, is a woodland setting for families and communities to connect with nature.

Kate's garden aims to highlight the benefits the natural world brings to mental and physical well-being.

Kate has made 'early years' intervention one of the cornerstones of her work after seeing how so many problems faced by adults she has met – such as mental health issues, addiction and social exclusion – stem from their childhood.

She and her co-designers were keen to recreate memories, such as wild bluebells, from her childhood.

In an interview with gardener Monty Don for the BBC, to be broadcast tonight, Kate says: 'I really feel that nature and being interactive outdoors has huge benefits on our physical and mental wellbeing, particularly for young children.

'I really hope that this woodland that we have created inspires families, kids and communities to get outside, enjoy nature and the outdoors, and spend quality time together.'

It is understood Kate, who wore a colourful patterned dress and wedges for the outing, wanted to show her children the finished project on Sunday, so they could see what she has been spending time working on.

Kate has previously said she has 'fond memories' of being outdoors as a child and is passing that passion on to her children.