Many, many books in libraries are problematic – and we’re talking about issurei de’oiraisah. If you don’t let your children read books with adult references/content [and you shouldn’t!], then don’t let them go to the library unsupervised. If your child is a ‘bookworm’, the chances are he will be reading things he shouldn’t.
I can tell you from personal experience – I was always a very avid reader; when I picked up a book, I was in a different world until I put it down – which certainly wasn’t until it was finished. My parents were very careful what they allowed me to read; my mother read each book before I was allowed to read it. I never bought a book into the house that my mother hadn’t okayed. Yet, while browsing in the library for new reads, I came across things that my mother would be horrified to see me reading. Books that I regret ever picking up.
The only way to truly safeguard your kids – or yourself – from reading ????? ?????? is to only allow books that you already know to be Kosher, either from your own reading or from someone you trust. To allow your kids to read unsupervised in the library is something very detrimental to their yiddishkeit, and certainly ossur.
(This isn’t just my opinion. Almost every guy I know who used to read secular books agrees that they have ‘stumbled across’ – or read – books they wouldn’t read loud in front of their mothers [which is a pretty good guideline of what is considered Kosher])