"In this situation" obviously means many different things - but my observation is that most families with mathematically very advanced children have at least one parent who is also mathematical enough to do the tutoring at least until the teen years. Of course there will be exceptions, and of course even if there is a capable parent there might be logistical reasons to outsource. Fwiw, with my mathematically advanced child, this is an issue that has faded - when he was young, it seemed clear there was no advantage in having someone else do what we could do, but we thought we might need a tutor later; now he's 10, it's clear that by the time he needs more than we can provide he'll be adept enough at learning from books/correspondence/random academics that it won't be a hard problem. I think the need you have may actually be rare.
