I'm no theologist
,
but I think it's a maturity issue as well as one of faith and reading level.
There's a lot of dirty stuff in the Bible; there's no denying that. There are passages that I don't think kids should read until they understand the facts of life, death, and everything in between.
If it were my kid (which I don't have any, so this is coming from nowhere in particular), I'd keep him/ her on the children's bible until he/she was mature enough to understand the big stuff.