It doesn’t really matter if the parents see the fine as “a fine” or an unaffordable, automatically billed service if it’s priced right.
That said, if lateness is a problem, it’s impeding on the workers’ personal lives, which can’t be good for anybody, so I don’t mind exploring how those workers could be helped. I assume more upfrontness from the business office, and parent education, would go a long way, similar to other institutions.
(I mean, I agree it's a big problem. I also dislike haggling, and I think I would have done much better had i outsourced that function when I ran a business, even if I paid a lot to that person)
All that said, a sufficient guilt trip might be more effective than a fine in any case, and is something that a caretaker could probably provide.
If I am late to give back a book that I am still reading, I do not mind giving it back one or two weeks late after I am done with it.
The fine is actually considerably cheaper if you compare with the upfront cost to be able to use the library.