Well it's still illegal to show pornographic content to underage teens and children is it not? I'm no expert on law, but I find it reasonable to believe that the librarian could be held responsible for allowing children to wander into a pornographic section.
The only legal directive librarians follow is CIPA: http://en.wikipedia.org/wiki/Children's_Internet_Protection_Act, which basically instructs them to give due diligence in filtering computers children have access to. That's it. They only need to follow that if they get certain money from the federal government. Librarians (outside of conceivable special programs) don't have any other requirements with regards to what kids do in libraries. As previous posters said, they're not babysitters. If a parent leaves a child anywhere in a library unsupervised, then that creates an unsupervised child in a library.
Most children's librarians I know (which is a fair number) wouldn't have a problem actually helping kids find any material in the library they asked for, no matter the obscenity level. Some do, but none of the libraries I've lent from have even restricted R-rated movies from anyone who wanted them. It's ALA policy for no books to be restricted by age, and yes, many libraries carry some very racy stuff.
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