Royals Princess Kate Plays Monster on Secret Play Date with Prince George – in Matching Mommy-and-Me Outfits! "She was flailing her arms about and chasing him," an onlooker tells PEOPLE By Monique Jessen Published on June 3, 2015 12:10 PM Share Tweet Pin Email Photo: John Stillwell/WPA Pool/Getty 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. Fashion-wise, the pair were sweetly in sync for the cold, wet morning: “They had matching waxed-green coats on, similar to a Regatta [brand], and dark jeans with boots. They both looked matching!” With Prince William getting ready for a starry polo match and one-month-old Princess Charlotte nowhere to be seen, George was clearly enjoying having his mommy all to himself. “He looked like such a happy lad, being chased by his mom and enjoying the swings, slides, roundabouts – they were going on everything. They looked just like any other mom and toddler. ” As for Kate’s antics, it reminded the observer of a certain Disney film. “It was a bit like Monsters, Inc.,” says the park-goer. (We’re thinking more along the lines of Shrek‘s Princess Fiona.) The observer also adds that Kate showed no sign of newborn exhaustion: “They both looked really well.” It’s hardly the first private play date for the two: They were also spotted having fun on the swings near the family’s country home, Anmer Hall. Want to keep up with the latest royals coverage? Click here to subscribe to the Royals Newsletter.