They have ratings on them. I'll give you a hint. If you're kid doesn't fit within the range of the rating then you shouldn't buy it for them.
This is no different that how parents should treat other forms of media entertainment. Some old folks can’t wrap their head around the idea of a video game being more robust than Pac-Man, thus they don’t pay enough attention to the ratings. I will say that my gf hates my violent mature-rated video games and has made it clear, so any future kid of mine won’t be getting near them.