But yeah, hitting a drive volley on these shots if the pusher is indeed just floating them back with underspin is the best way IMO to tackle such an issue. That is odd though that his opponent chose to float the ball using a slice. Good pushers will load up on the topspin from my experience, which is wayyyyy more difficult for younger players to cope with.
Yeah, usually moon-balling is very effective at that age because it exploits their height. To the little pusher's credit, though, he was very good at underspin floaters, and it was troubling because my players (in Singles and Doubles) have had little exposure to that spin, and it really exploited their movement and anticipation, which most kids struggle with at that stage.
I'm with you on the drive volley, and appreciate your logic and reasoning behind it. It's definitely what I'll work to implement the next time I meet with them.