A lot of it comes down to knowing the community. I don't take on clients if they don't already have a reputation in the ERP community because if they aren't an established person, as you said, there would be a non-zero chance of interacting with a minor if you're blindly going after anyone who tries to initiate. For me there has to be a 0% margin of error or I'm wholly uncomfortable with even the potential. For people that I don't have any trust in we just guide them to the bar/lounge RP and internally put a ban on taking it any further than that unless we can 100% feel comfortable and confident they are not a minor. Normally what happens is they request someone and we'll just brick wall them with "Well nobody is available right now. Let's just focus on progressing our characters storyline a bit more." I'd rather pass over 100 genuine adult clients to avoid that 1 minor who's pretending to be an adult.
Another thing is going with established venues minimizes that risk because there is no way FC leaders are going to destroy their reputation, their FC and their members by introducing that level of risk to their clients. Not to mention a lot of FC's may have 'anchor members' or people known across the greater ERP community for their skill, artwork, writing etc. (Not a real term for it, that's what I just call it) I know we have several members that have alts sprinkled around other FC's that are very well known. If they have you as a client it's kind of the gold standard of writing.
There was one FC leader on our server who willfully interacted with a minor, when the FC leader found out they were under 18 they continued to pressure them into ERP acts. That minor went to another FC leader with evidence on what happened, and that person got a pretty miserable reputation after the fact (Not that they were in a stellar position before hand but that's another story entirely.) GM's got involved, I believe the police were called there was this whole drama fiasco, and a good amount of people banned him from their events/venues/discords for that reason alone. They even got mocked in NN for 'grooming' which was a very well deserved reputation. The ERP community has its problems but I like to believe we draw a hard line on that.
I truly believe that people in the community are of the mind that involving children even accidentally is abhorrent. That's nightmare fuel stuff.