I honestly don't think this law is related to sex offenses. I think it's related to the fact that adults can easily push children away, and then the taxpayer's money is not being used for the intended purpose. Adults have plenty of places they can go. Kids, not so much.
(Similarly, if you're 40 years old, you can't go to the public kindergarten. It's because the program is designed to educate 5-year-olds, not 40-year-olds. Nothing to do with sex, it's merely practicality.)