Kate played with schoolchildren in her woodland-themed garden at the event, helping youngsters connect with the natural world.
The hands-on duchess, dressed casually in a white blouse, camel-coloured culottes and white trainers, climbed up a wooden ladder into the garden’s centre-piece – a high platform tree house, and sat on a tree stump during her visit.
She helped children from local London schools build miniature rafts to race down a waterfall feature, roasted marshmallows on a campfire, and also chatted to them about nature.
Speaking to teachers, Kate was heard to say: “It’s really important the kids start to understand nature, and care for it, so they can protect it as well.
“With Instagram everything is so visual. It’s nice to sit and feel.”
The royal spoke to mother Alison Shockledge while touring the Back to Nature Garden, designed by Kate and landscape architects Andree Davies and Adam White of Davies White Landscape Architects.
Ms Shockledge said: “She’s a mum. She was talking about it from a mum’s perspective: put your devices down, let’s go out.
“Be relaxed with your children, let them get muddy.”
Kate urged children to smell flowers, and enjoy the scent of the open campfire. She said: “Have a smell, they smell amazing.”
The duchess was pictured on a rope swing during a previous visit to the garden with her three children.
When she passed the swing again she told the schoolchildren: “It’s so good.”
The duchess also chatted to Colette Morris and Rebecca Beale.
Ms Morris said: “She said children played very differently. In a way she didn’t anticipate.”
Ms Beale said: “Children are often sat still looking at screens. She said it was important to be multi-sensory.”
WATCH: Kate and William Share Adorable Family Photos at Chelsea Flower Show Garden
Kate shared a laugh with one of her assistants who tripped into some mud in the woodland garden.
As the pair laughed, the royal asked: “Are you OK? I thought I was going to do that before.”
The royal finished her time in the garden by writing a note with one of her memories of enjoying the natural world. Her note read: “Boulder hopping in the Lake District.”
On Sunday, Prince George, Princess Charlotte and Prince Louis spent almost an hour playing in their mother’s garden, as shown in six candid images released by Kensington Palace.