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It's not every day that a princess pretends to be a monster.

In her first public sighting since welcoming Princess Charlotte on May 2, Princess Kate visited London's Holland Park last weekend with her 22-month-old son, Prince George, where she joined the ranks of moms to play that time-honored (and tiring!) game of chase: monster.

On Sunday morning, a playful Kate, 33, was spotted entertaining her rambunctious toddler at the picturesque park, a 10-minute drive from the family's residence at Kensington Palace. "It looked like she was pretending to be a monster, flailing her arms about and chasing him," an onlooker tells PEOPLE exclusively.

"She was play-acting just like all mums do and he was running around loving it – he looked very happy," the onlooker adds.

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