You know what, people mature at different ages, and if they made sure their kid understands that violence is bad and shouldn't be practiced in real life, as well as if they sat through the game with him and explained that the stuff that happens in there isn't real and shouldn't be made real.
They could've also had him acting, then took the game away from him after the video was filmed. There are several factors that could change whether this video truky shows bad parenting or not.