Kate spent time playing with schoolchildren in the woodland-themed garden she helped design at the event.
The duchess, dressed casually in beige culottes, a £255 broderie anglaise blouse and white trainers, climbed a ladder into the garden’s centre-piece - a high platform tree house, and sat on a tree stump during her visit.
The 37-year-old helped children from local London schools build miniature rafts to race down a waterfall feature, roasted marsh-mallows 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 Duchess spoke to mother Alison Shockledge while touring the RHS 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 from Christchurch school in Battersea and Ashburnham Community School in Chelsea to smell flowers, and enjoy the scent of the open campfire. She said: “Have a smell, they smell amazing.”
The mother-of-three 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.”
Kate 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 added: “Children are often sat still looking at screens. She said it was important to be multi-sensory.”
The Royal finished her hour-long visit to the garden by writing a note with one of her memories of enjoying the natural world. It read: “Boulder hopping in the Lake District.”
Yesterday, Prince George, Princess Charlotte and Prince Louis played in their mother’s garden, as shown in six candid images released by Kensington Palace.
Kensington Palace announced in January that the Duchess would be helping with the design of this year’s RHS feature garden.
News of her involvement sent ticket sales surging, with all 168,000 sold out last week ahead of the show’s opening to RHS members tomorrow and the general public on Thursday.
Among the other guests at the show this morning was TV presenter Nicki Chapman, 52, who has covered the event since 2006 and revealed at the weekend that she has undergone surgery to remove a brain tumour “the size of a golf ball”.