this does not seem like the kind of website that it would be acceptable to have someone claiming to be so young
Given that when you join you declare that you are over 13, and the site's terms of service explicitly state such an age requirement, I would guess that "her" membership is clearly in violation of that 1998 law.
I agree that it is entirely possible that the poster is not a 10 year old child but actually a rather older person, probably male, who gets a certain kind of gratification, too gross to describe here, from such impersonation. Or else the person is really a 10 year old girl, in which case she runs the risk of providing gratification for such an older person. Either way, mods please note!