Video report by ITV News Royal Editor Chris Ship
The Queen was pictured smiling as she inspected the garden created by her granddaughter-in-law for this year's Chelsea Flower Show.
Wearing a light green coat, Elizabeth looked at the garden with William and Kate before touring other gardens at the flower show.
Chelsea is a firm fixture in the Queen's calendar. Her Majesty is known to enjoy the gardens at her palaces - so a trip to the show was no doubt an engagement she was looking forward to.
The Duchess of Cambridge's garden aims to encourage children to spent more time outdoors and photographs taken at the Royal Horticultural Society (RHS) Back to Nature Garden show Kate's three children playing in the outdoor space.
In new candid images earlier released by Kensington Palace, the family can be seen enjoying time together in the garden.
In the footage Prince George is asked by dad William "how many marks out of ten" the young royal would score his mother's garden.
George, swinging on a rope in the garden, responds "twenty", to which William remarks: "I think mummy's done well."
A playful Charlotte can be seen dangling their legs over a stream, while an excited Louis runs along a wooden path - the first moving picture of the youngest Cambridge since his christening in July.
On Monday morning, Kate was joined in the garden by some local school children. She was pictured toasting marshmallows over an open fire and talking with the children about the importance of enjoying the outdoors.
The duchess got herself into a bit of a sticky situation when trying to get one of the marshmallows into the mouth - managing to get most of it stuck on her fingers.
Whilst speaking to school children, the duchess managed to find a boy who was adamant he enjoyed doing homework instead of watching television.
Speaking about the garden she helped to design, and which opens to the public on Tuesday, the Duchess of Cambridge said her Back to Nature garden was designed to be "a natural space, a really exciting space for kiddies and adults alike to share and explore".
Kate added that spending time outdoors and enjoying nature is hugely beneficial, especially for children, for their physical and mental well-being.
She hopes the garden inspires families and communities to "get outside, enjoy nature and spent quality time together."
The Duchess pointed out the "amazing scientific research... about the importance of being outside and the benefits it has...particularly on young children, on laying the foundations for positive physical and mental well-being".
She said both life and physical skills could be learnt by young children as they played outside, something she "hoped" her "garden brings".
Charlotte, dressed in a floral flock, and one-year-old Louis can be seen trying out a swing seat and George can be seen adventurously clambering across the stream.
The garden has been co-created by landscape architects Andree Davies and Adam White and the RHS.
Kate's garden aims to highlight the benefits the natural world brings to mental and physical well-being.
The duchess said in an interview for the BBC: "I really feel that nature and being interactive outdoors has huge benefits on our physical and mental well-being, particularly for young children.
"I really hope that this woodland that we have created really inspires families, kids and communities to get outside, enjoy nature and the outdoors, and spend quality time together."
The space, which Kate hopes will inspire families "to get outside" and "enjoy nature", features a tree house, waterfall, rustic den and a campfire as well as tree stumps, stepping stones and a hollow log for children to play on.
Prince William accompanied his wife and three children to the garden and showed his playful side by climbing into the tree house with his two eldest children.
George, Charlotte and Louis contributed to their mother's creation by collecting leaves, moss and twigs over the past months which were then incorporated into the surroundings of the campfire - hazel sticks collected by the family were also used to make the garden's den.
It is understood Kate, who wore a colourful patterned dress and wedges for the outing, wanted to show her children the finished project ahead of its opening, so they could see what she has been spending time working on.
Kate has previously said she has "fond memories" of being outdoors as a child and is passing that passion on to her children.
The duchess has been at the site almost every day in the five days leading up to the opening of the event and has been closely involved in the project from the very beginning.