An onlooker told the Sun Kate said: “They insisted on getting up early so they could come and water the garden.
“They are excited to be back.”
Kate and the children spent around 20 minutes at the Royal Horticultural Society “Back to Nature” garden.
But the show which opens at 8am do not allow children under the age of five to the gardens.
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Kate secretly took her children back to the garden she co-designed (Image: GETTY/ Kensington Palace)
Princes George and Louis as well as Charlotte were taken to the plot (Image: Kensington Palace)
This does not apply to Prince George who is five – but Charlotte and Louis are not old enough to be allowed into the exhibit.
Prams, pushchairs and babies carried in arms are also not allowed, according to the RHS website.
A spokeswoman for RHS told Express.co.uk: “We are incredibly honoured to have HRH The Duchess of Cambridge co-design the RHS Back to Nature Garden at this year’s RHS Chelsea Flower Show, helping inspire children, families and communities to connect with and enjoy nature.
“The Duke and Duchess visited the garden with their children on an evening before the show opened to the public.
The children reportedly wanted to water the garden (Image: Kensington Palace)
“The age restriction during show week remains the same and is in place for health and safety reasons.
“Children of all ages will be able to come and enjoy the RHS Garden co-designed by The Duchess of Cambridge at RHS Hampton Court Palace Flower Show in July and RHS Garden Wisley in the autumn.”
Kate may have got away with allowing Louis and Charlotte in the garden as different rules apply to show exhibitors.
She previously brought her young children to explore her garden before its official opening.
Kensington Palace have released six adorable images of the family playing in the garden.