thats a big question. first its interesting you go straight to best in the country when someone asks for a coach recommendation. second, this is kind of like a ball recommendation question when someone asks whats the best ball for my league.
is the kid an advanced 10 year old? is this their first year bowling a league? have they ever been coached before? do they know anything about bowling fundamentals? what is their aptitude for bowling? are they a smart or "average" 10 year old (sorry parents there are smart and dumb kids out there).
i second coach slowinski. hes got to be top 5 at least. although i dont know how he does with rudimentary and young kids coaching. ive only seen him with high school age and older and more technically advanced players