IronWing and Broheim are right. Being molested by an unfamiliar person is pretty much at the bottom of list of things which statistically threaten children. Library's actions are just stupid.
it's probably because they don't want people looking at pornography on the children's computers
I bet some of the older children are actually the most likely library porn-browsers...
Even if that were the motivation, the library could simply restrict the children's section computers for children. Not the books, not the section.
If that kind of idiocy was practiced at my library, I'd try to loan a children's book and see how that works. I suppose they would then have to escort me in to pick a book or fetch me one that I want. Either way it's hassle for them. If I kept loaning one book every time I go, most likely they would eventually make an exception and wave me through without oversight, thus proving the rule had no real point in the first place.
Then it would be possible to write or meet the management to ask about the policy, and to go public with their response which will inevitably be badly reasoned.