A stupid point worth mentioning: Since libraries are government funded, you may need to be careful about what you give them for consumption by children. For example, since you bring up Captain Marvel, there are some...unfortunate incidents (Whiz Comics #12, say). And...well, there's Steamboat. And Captain Nippon.
I mean, I'd be perfectly happy letting kids read such things, because I don't think any reasonable child would look at it and say, "oh, well, I should go ask my Asian friends when they fixed their buck teeth." But one activist teen stumbles across it, and you're run out of town on a rail.
(As for copyrights, libraries are allowed to carry things that are copyrighted. They're even allowed to make scans of copyrighted works available provided that the intent is for viewing and it'd be otherwise difficult to acquire a physical copy. The rule is usually meant for replacement/preservation purposes, where the library does own a copy, but I don't think it's exclusive. Besides, it's the government, which technically has a license to all Intellectual Property in exchange for granting you those rights, though I don't believe any administration has ever leveraged that detail.)