There is no digital equivalent of "flash an ID card and be done with it" in the surveillance state era of the internet. Using a CC is the closest we have and even then you're giving data away.
Additionally, the laws I've read mandate that no data be retained, so you have stronger legal protections than typical credit card use, or even giving your ID to a store clerk for age restricted purchases (many stores will scan it without asking, and in some states scanning is required).
I expect most forums or discussion groups in practice actually don't have child-inappropriate content, and already moderate such things because the members don't want it.