The first book I ever read was Samuel Delaney's Distant Stars. I had a vague idea at the time that I didn't know all that was going on. Looking back, I'm amused at how much I missed: I missed that he was a black science fiction writer (an all-too-rare combination), I missed the vague sexual themes (I was nine), and I missed that the writing wasn't that good (decent, but Delaney's done much better).
I mention it because while part of me thinks it's silly to give that book to a nine-year-old, another part of me says "Served me well enough." The book made me struggle, left me confused at times and exhilirated at times. Most of all, though, it made me want to try again.
It made me read.
So now I'm inclined to imitate the neglect of my parents and librarians--no adult ever denied me a book. Go struggle with the books, I say, it's good for you and it strengthens you in a way that nothing else in this world will. Beside, if they meet something for which they are truly unready, they won't be frightened or disturbed. More likely, first it will be confusing, then it will be forgotten.