COPPA only applies if you're collecting, using, or disclosing personal information FYI. Facebook/YouTube have set in age limits that prevent you from even inputting any ages below 13 just to avoid the issue entirely, but as Imgflip collects no personal information , they're fine to be here. It's against their terms of use for sure, but I'm not sure if it's illegal per se.
I will say though that having had to deal with enforcing COPPA on one of the sites I used to manage, it was such a PITA, it's really no small wonder why most sites simply choose to prohibit the 12 and under group entirely. Even on a site this size (one of which COPPA would apply, anyways), you still need a small team of individuals dedicated to combing through all the comments to find them. As long as you're not knowingly disclosing personal information, you're exempt, which wound up being our "saving grace" -- as long as we enforced the rule that 12 and under individuals were forbidden from sharing personally identifying information, we didn't have to deal with it any further.