It happens if your kid knows how to walk, and it's not a hack or anything it happens alot in my game.
Really? That's funny, I've never seen my Sims kids do that before in all the years I've been playing the game. He wasn't even in there for that long, he climbed out just as I sent my Sim downstairs to take him out. I thought he was a genius for a second there, haha.
Thanks for the response, though, I was completely baffled. I thought I'd downloaded something I never meant to.