CADad,
How big is their stride? Are they lunging? I've been working w/ some 10-12YOs the past few weeks and I see these issues in 3/4 of the hitters. Their strides are 18+", their hands drop during the strides, and they never firm up the front leg to stop all that momentum. Even my son often takes too big a stride.
What I tried this week w/ my son is some pure no-stride practice. (I'm not looking to start a war over this, as even I don't think he needs to hit like this, but as a practice tool...). Removing his stride and focusing on keeping his hands up (or rather, moving them up a bit during heel lift) gave him the knowledge/confidence that he can still hit the ball hard w/ a "quiet" swing.
Then, when he went back to striding last night vs. pitching and machine, our cue was "short stride, hands up" and he absolutely crushed line drives at will. One sequence he told me about over breakfast today was he "hit a high linedrive left, then a low one left. Then a high one right, then a low one right. Then he hit the L-screen, then the back of the cage. All IN ORDER." Now, I'm not saying he was aiming in these areas before the pitch, but NTL, just being able to rip off that many line drives in a row w/o popping up or pulling outside pitches into the ground, etc., is great progress.
My point is... what I often see w/ upper body hitters (kids anyway) is also very big strides and too much lunging. Sometimes if you can convince them that they don't need to do so (that their power can come from elsewhere and w/ less effort even), you might make some gains.
Good luck,
Sandman