I'd hate to be a school librarian who would have to keep track of every parents' weird book prejudices. How about they can give their own kids a list of all their rules and no-read books, etc, and if the kid doesn't follow their rules, then that's between the kid and parents.
Actually if it is so important to parents and it really is their responsibility then allow them to come in and type the names of the books they do not want their child to see in the computer.
That works also, I think my way is best but I really do not care - I do not believe in banning any books.
You know they have computers now and I assume children still check out books so you know who has them? So the computer would flag any books for a particular child - no real effort other than putting the info in the computer.