Prince George doesn't have to worry about his little sister, Princess Charlotte, stealing the spotlight. The royal toddler had some mother-son fun on Sunday in London's Holland Park. The pair were seen energetically playing a monster game, despite the rainy weather.

At about 10 a.m. on the "wet morning," Kate and George were the only two in the play area in the London park and seemed to get caught up in their game, an onlooker told Hello.

George was "really smiling and laughing" as his mother "chased him, waving her arms around pretending to be a monster," said the passerby. "It reminded me of the film 'Monsters Inc.'"

"They were running around on the rides, the swings, roundabouts and slides," added the source. "They were on their own. Just the two of them."

Middleton and her son were accompanied by two royal protection officers. Mother and son were wearing "matching waxed-green coats, dark jeans and boots."

The 22-month-old heir should learn early: you're never too old - or too royal - for a playdate with your mama.