My oldest son has Asperger's, and has a very difficult problem with rage, violence and a few other things. Even in reading books, I try not to let him be exposed to too much violence. This was when he was 12, now that he is 17 - of course he reads more. I just like to always read the books, too - so that I can discuss them with them.
As far as my daughter goes, she's only 11. I knew that the Twilight series was a vampire and romance series. I didn't know it was actually geared for younger readers. Being an Anne Rice and Laurell K. Hamilton fan myself - I wanted to make sure there were not any sexual scenes in the book like those. She's not ready for THAT explicit, yet.
My kids know that all of our books in our library downstairs will one day be available to them, if not now. But being major SciFi/Fantasy and Horror lit readers - my husband and I have some books that are just not appropriate for their ages.