Inside Prince George's Playdate with Princess Charlotte and Mia Tindall: 'They Are All Live Wires'

"They were all just messing about and playing and joking while their parents chatted," an onlooker tells PEOPLE

He may be the future King, but Prince George was just like any other little kid over the weekend as he enjoyed a playdate with his cousins and younger sister, Princess Charlotte.

Five-year-old George, whose blonde locks have gotten a little darker, shared a cute moment with his cousin, Mia Tindall, 5, on the fairgrounds of the Burnham Market international horse trials in Norfolk on Friday.

The royal peered down at Mia, whose mom is Zara Tindall (Prince William’s first cousin!), as they appeared to compare their heights.

“It could have been they were measuring up against each other. But they were all just messing about and playing and joking while their parents chatted,” an onlooker tells PEOPLE.

The royal playmates, who are just six months apart, had fun with their parents — Prince William, Kate Middleton and Zara and Mike Tindall — before George joined his sister Princess Charlotte, who turns 4 on May 2, for a ride on the bumper cars and slides.

<a href="" data-inlink="true">Prince George</a> Mia Tindall Burnham Market Horse Trials

Zara and Mike’s youngest daughter, Lena, who turns 1 in June, was nearby in her stroller as the older kids periodically went to check on her throughout the day.

“They had been watching Zara (an accomplished equestrian) competing in the showjumping, from the top of a hill and then all ran down a hill together,” the onlooker continues. “They were playing together all. Mia will make friends with anybody!”

Their fun and friendly relationship matches that of their parents, Prince William and Zara. The older royal cousins are similar in age. Over the decades, they’ve often been seen at the same royal — and non-formal — occasions from Trooping the Colour to polo matches with their kids.

*PREMIUM-EXCLUSIVE* <a href="" data-inlink="true">Prince George</a>, <a href="" data-inlink="true">Princess Charlotte</a>, Mike Tindall and Mia Tindall are all seen enjoying a day out at the Burnham Horse Trials in Norfolk

“Mia seems more outwardly going than George,” the source adds.

William, who is one of Mia’s godparents — gave Mia a ride on his shoulders at one point, while Zara’s husband, Mike Tindall took George on his shoulders as the young prince played with a toy sword.

“They are all live wires, with loads of energy,” the onlooker says.

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