That's why I'd ask for, and keep, a recognized legal proof of age document (such as a driver's license, passport, or birth certificate.) It lets me see that they have these documents, that they have proof of these documents, and gives me paper to cover my backside in case they try something hinky.

If you've done everything reasonable within your power to ascertain proof of legal age once doubt was cast on your previous proof (the person signing up with your service and agreeing with your TOS), then you should be safe, and more importantly, your community should be safe. Granted, there will always be a risk that someone somewhere will pull some obscure legal clause out of their backside (or their lawyers') and try and make you the fall guy; or some parents who are either non-attentive or just have a very, very clever kid (or other variations). But if you've gotten proof of legal age, and you requested this proof soon after seeing someone claim that they were *not* actually of age, you've done everything you can to protect everyone involved.

If the parents claim they were unaware of the child's activities, but you made a good faith effort to legally establish age, what else can you do? You could put a policy in place that, say, if someone publicly posts that they're underage several times, then despite legal proof of age you can delete their account and not refund any monies, based on their casting unreasonable doubt on your community's security. Check with a lawyer before you try that, though - while it does look like it neatly shields you from COPPA indemnity, it might get you into other forms of hot water.

You seem to have a LOT of pretty good legal questions. While you're getting some good advice, you're also getting advice from people who aren't legal professionals and who might not be familiar with the laws of your area. I recommend that you make a list, gather all information you've received, and make one consultation-appointment with a lawyer. It might be costly; but you'll get a whole bunch of questions answered in one go. Prepare well, and you'll definitely get your money's worth. Knowledge is power.