Children's libraries and children's sections have always been policed. I was a big Enid Blyton fan but my local library didn't think they were "suitable literature for children" so they just didn't have them.
I also agree with keeping certain books out of displays.
Children are people, people who are exploring their surroundings. It is up to parents to police what is suitable for their individual child. A library that displays books not suitable for children is over stepping that mark.
Children learn about the world around them in child terms. We tell small children not to talk to strangers, even if they do give you sweets and tell you they have some puppies.
We do not go in to gory details of what has happened to some children who have been abducted, assaulted, tortured and killed.
Children don't need that message, they need, "Don't talk to or go with strangers".