However, it wasn't long before Charlotte kicked off her shoes to run around the pitch barefoot.
Prince George wore a navy blue polo shirt with Khaki shorts while his sister Charlotte wore a pink floral frock.
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Of course, the Duke of Sussex has had plenty of cheeky moments over the years - the cute photo of him sticking out his tongue while in mum's arms at Trooping the Colour is an enduring one.
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It looks as though the toy gun came as part of a classic "police set" as the youngsters can also be seen with a plastic baton and some handcuffs and Prince George is known to have a fascination with the police and emergency services.
The event marked Prince Harry and Meghan Markle's first appearance together following their rumored honeymoon. The cheeky 3-year-old at one point showed off her handstand skills putting her mom into a fit of laughter.
Prince William was competing in the Maserati Royal Charity Polo Trophy in order to raise funds for both Royal Marsden Cancer Charity and Centrepoint, a foundation aimed at helping homeless young people.
But photos immediately after show Kate was actually running to greet England polo player Malcolm Borwick, who appeared to be beckoning her from beside the white marquees a few metres from the grass bank.
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Kate kept a close eye on her little girl and oldest son, who spent much of the day chasing after each other and their cousin Savannah Phillips.
The incident brought back memories of the 2016 Trooping the Colour when William, 36, was scolded by Queen Elizabeth to stand up.
The next time the public is likely to see Prince Louis will be at his christening later this summer.
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