Kate Middleton & Prince William Take George, Charlotte & Louis for Sneak Peek of Special Garden

Kate Middleton’s show garden had the ultimate road test on Sunday when her three children George, Charlotte and Louis got to play in it.

Royal mom Kate and husband Prince William took their children for the outing on Sunday afternoon. The family sat in a den that they had all helped source wood to build, while George and Charlotte took off their shoes and dipped their feet in the stream beneath a waterfall, and little Louis, 12 months, was seen walking and brandishing a stick.

Kate, 37, had has spent much of the last week putting the finishing touches to her show garden — so she was keen to show them the fruits of their labor at the famous Chelsea Flower Show in London.

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Her children had also helped to pick moss and twigs and gather sticks of hazel used in the creation of a den, that mirrors one from their country home Anmer Hall in Norfolk. During the afternoon outing, 5-year-old George, who dressed in a H&M polo shirt, and 4-year-old Charlotte, who wore a floral dress by Rachel Riley, clambered into the stream, checked out the treehouse and got to see a hollowed-out tree trunk that is used as a play tunnel.

“George and Charlotte went up into the treehouse,” says a royal source during the 45-minute stay.

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And, matching mom Kate’s go on the swing (pictures of which she released Saturday), William with Louis, George and Charlotte took turns on it. Princess Kate loved showing them around, the source says. “It was a lovely happy, really lovely afternoon,” the source adds.

The Duchess of Cambridge collaborated with the Royal Horticultural Society and designers Andrée Davies, 55, and Adam White, 45, to complete the “Back to Nature”-themed garden as she highlights the benefits of playing outdoors.

She has been a hands-on co-designer and visited several days last week and on Sunday morning, clocking up more than 20 hours on the site in recent days, insiders say. “The duchess joined us as it all started coming together. It’s addictive and once you’ve got the bug you can’t stay away,” White says.

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Kate joined them on away days to source the trees from nurseries and chose forget-me-nots (the favorite flowers of her late mother in law Princess Diana) for the woodland floor. The treehouse, comprising 9-tons of wood, had been transported from around 100 miles away the designers revealed Sunday. There is a bow and arrow on the floor by a campfire and reeds that can imaginatively be used to create handmade natural boats to float down the brook.

“Hopefully when people visit they will find their inner child,” Smith told reporters Sunday. “That could be remembering building a den or toasting marshmallows on a fire.”

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Davies adds the design was meant to be “really immersive. Children spend a lot of time indoors” and that the garden presented “a gentle reminder” about the benefits for “both physical and mental wellbeing.”

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She tells garden expert Monty Don in an interview, which airs on Monday, May 20 at 7.30pm on BBC One, “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 really inspires families, kids and communities to get outside, enjoy nature and the outdoors, and spend quality time together.”

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A day earlier, Kate had shared images of herself on a swing in the garden as she highlighted her drive for mental wellbeing among kids. “In recent years I have focused much of my work on the early years, and how instrumental they are for outcomes later in life,” she said.

“I believe that spending time outdoors when we are young can play a role in laying the foundations for children to become happy, healthy adults.”

The new pictures — taken by Matt Porteous — were released by her office at Kensington Palace late Sunday.