June 13, 2018 02:16 PM

Even a future king needs a little playtime!

Prince George showed his playful side during his dad, Prince William‘s charity polo match at Beaufort Park last Sunday.

One day after attending Trooping the Colour, Kate Middleton took George, 4, and Princess Charlotte, 3, to cheer on their dad. While Charlotte seemed to steal the show by rocking pink sunglasses and doing headstands, George was also in on the fun.

“He leapt right into it. They first thing they did when they left the car was slide down the hill on his bum,” an onlooker tells PEOPLE. “He was peering over some hoarding around the side of the pitch, grinning at the others, who were messing around with him.”

Prince George
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George, who usually appears to be on the quieter side (he hid behind dad William outside the chapel following Prince Harry’s royal wedding), was full of energy and excitement as he darted off as soon as his feet touched the ground.

Prince George and Princess Charlotte
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Prince George
Max Mumby/Indigo/Getty

He also goofed off with his older cousin, Savannah Phillips, 7, who famously covered his mouth while they were on the balcony for Trooping the Colour, which honors Queen Elizabeth’s birthday.

Peter and Autumn Phillips’ 7-year-old daughter started the fun by pretending to sing the national anthem, but when George joined in she “realized it was probably not what they should be doing,” so she placed her hand right over the future king’s mouth when he started to giggle, a royal observer tells PEOPLE.

James Whatling/MEGA

After all the polo fun, Kate tried her best make sure the kids were tired out by the end of the day before they got back to the car.

“Kate is a fantastic mum,” says the observer. “She was calm with the children.” Using a British phrase for “run loose,” the onlooker says, “She let them have their head.”

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