I always wondered the same thing.
Kids caster in horror movies always bothered me a little.
Danny and the chopped-up Grady Twins in The Shining; Drew Barrymore in Firestarter; the dead child in Pet Sematary; Isaac in Children Of The Corn.
But you should also consider the guys who are making the movie are all professionals and take care of the kids they cast all the time.
It's debatable but I believe it's just for entertainment so it's alright.
You do make a really point; but another thing is that them BEING in the movie gives them an excuse to watch it. For example, Isabelle Furham in Orphan, claims she watched it a bunch of times, and she's 13. And Abigail Breslin in Little Miss Sunshine watched it, and she was only 8! But I guess it all depends on parents to. My friend watched Friday the 13th in theaters, and she's 11!