Good work - Labelling someone is really going to help them hear your point of view… And that literally the precise opposite of what I am trying to say, which is that the posted rules are the thing that applies here. If loud kids are not prohibited the signs saying “be quiet” are no longer there, they are not technically in the wrong.
FTR, I totally think the parents are actually the assholes here, and not OP. My point is that if OP did indeed confront them, and he was insulted, he has little recourse but to walk away or find someone in charge of that space to deal with it. If there are definitively clearly posted rules, then point that out to them would help. Alternatively, talking to the librarian in charge is the next step.
I read OPs post more about this being a more common thing in libraries, and I hope that with enough complaints the loud kids will either stop coming, or learn to be more respectful (because yes, that is a parents job to teach their kids that). Unfortunately, I don’t think that will be the case as we have all but lost the ability to see someone else’s point of view.
And massively increased again in the last five years…