Kate Middleton and her family may live in a palace, but she wants her children outdoors in their garden “rain or shine.”

The royal mom says she encourages Prince George, Princess Charlotte and Prince Louis to spend time outside whatever the weather, believing “it encourages creativity, confidence.”

“Even a short amount of time — 10-15 minutes outside — makes a huge difference to both physical well-being but also to our mental well-being,” she says in a new interview on CBBC’s Blue Peter.

Kate’s appearance on the longest-running kids’ TV show in the world, which premiered in 1958, is part of her ongoing campaign to help encourage kids to enjoy gardens. And she has teamed with the show to launch a competition for viewers to design a sculpture to go in her Back to Nature garden when it’s installed at a new venue, RHS Wisley in September.

She has also created a special collection of outdoor activity cards to go with the garden. Two runners-up will also get to design their very own Blue Peter activity card to celebrate the unique garden.

In a charming segment with Blue Peter presenter Lindsay Russell that airs on Thursday’s episode, Kate takes part in a race to build a den, helps some kids create innovative plant pots by upcycling some old Wellington boots and joins in a hunt for creatures like newts and frogspawn in a pond.

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She declares that pond-dipping is “massively up my street,” and she joins some of the schoolchildren at Westminster City Council’s Paddington Recreation Ground to build a den that mirrors one in her Back to Nature garden that was such a hit at Chelsea Flower Show last month.

VIDEO: Kate Middleton Participates in Den Building on Blue Peter

Her garden will evolve as it is moved to Hampton Court Palace’s flower show in July and then onto Wisley in the fall, a royal source says. It will permanently stay at Wisley after it is installed.

Earlier this week, on an outing with Prince William, Kate told wellwishers in Keswick that she recently brought her children to the Lake District for a few days of walking and playing along the hills.

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“She said that her children love coming here and walking in the fells,” Jack Mumberson, 11, from Ireby Church of England school, told PEOPLE.

The competition is open from Thursday, June 13 until noon on Tuesday, June 25. For more details, visit www.bbc.co.uk/BluePeter.