In my opinion, books should never be banned. I also believe that it is up to parents to teach their children right from wrong so that if they do happen to come across something that differs from the values they were taught, the child will know the difference and the fictional will not so easily become fact. They will be able to form opinions for themselves, to think of the consequences of the characters in the books and draw conclusions for themselves when relating it to their life.
I must also say that as parents it is a tough decision to make. It can be difficult to choose what we read to our children, to help them choose what to read when they are too young to choose for themselves what is appropriate for their age and understanding. I think it is safe to say that most parents naturally wonder from time to time, "will my child understand this topic or will it confuse them," I know I do. So in a nutshell here is my opinion on the subject: