The wife of Prince William took the little ones around a treasure hunt and a bug-spotting activity before tucking into a picnic in the centre of the garden.
The Duchess arrived at Hampton Court Palace in a green, square-printed midi-dress from French label Sandro Paris, Harper's Bazaar reports.
Kate's Sandro dress retails for $319 AUD.
Famous for recycling her royal fashion, Kate stepped out in her favourite pair of $166 AUD Carina espadrilles by Spanish brand Castaner, which she wore last week for an appearance at a photography workshop in south west London.
The Cambridges official Instagram account, Kensington Royal, shared a series of sweet shots from the garden and explained the purpose of the Duchess' garden:
"As part of her longstanding work on Early Years, The Duchess believes that spending time outdoors plays a pivotal role in children’s future health and happiness. Her garden highlights how time spent in natural environments can help build the foundations for positive physical and mental wellbeing that last through childhood and over a lifetime," the caption read.
Fans of the royal family were quick to shower the Duchess with praise on social media, with one user comparing her to Prince William's late mother, Princess Diana.
"She's absolutely incredible with people! Its like she was made to be royal, I haven't seen anyone this fit for this role since Diana. The Duke must be incredibly proud."
"She seems so genuinely happy with the people she meets and in all areas her work, love her ❤️," another wrote.
"The Duchess of Cambridge is such a lovely role model. She has really grown into her role. Congratulations for being such a great advocate!" someone said.