Kids can hit balls in a drill all day long but there is no replacement for a live ball in a match-like setting. Critical to development is finding competitive matches, especially when kids get to be 14, 15, and 16 years old. I think that's the challenge you're facing as this is where kids learn toughness, how to play tennis and how to really compete.
The difference between a very good player and a great player often lies in one's ability to out-compete the opponent. In my opinion, this is a piece of the puzzle that is lacking in the US...not enough of this happening. We need to do a better job of organizing daily scraps with consequences to the loser. In other words, make the match meaningful even if it's something as silly as loser buys gatorade. It doesn't have to be a full 3 sets but simulate some kind of match. Kids will reach new levels with more competition but the current system of getting involved in a tennis program and drilling every day often prevents this from happening.
If you're looking for kids to hit with, Florida has many great options, particularly south Florida. You can reach out to people like Saviano, Dean Goldfine, ZMG (Juan Nunez), Martin Blackman - I'm sure any of these guys would be accommodating or at least point you in the right direction.
Good luck to you.