I think the optimal swing, and the position that puts your body in, naturally produces a slight uppercut; especially on lower pitches. That's fine. When I see extreme tilt however, like with Dunn, or Edmonds, the angle of the bat greatly decreases the margin for error imo. And hitting is hard enough without lessening your chances.
I would drill home "down through the ball" as an important mental cue, of course making sure the kid understands it doesn't mean chop down. Chopping down and slicing the ball with backspin is completely different then staying through the ball and creating forceful backspin.
Maybe you or others on here, who have coached kids of all ages could tell me. How do you think the younger ones would respond? Would they understand the difference?
"By common consent, Ruth was the hardest hitter of history; a fine fielder, if not a finished one; an inspired base runner, seeming to do the right thing without thinking. He had the most perfect co-ordination of any human animal I ever knew." - Hugh Fullerton, 1936 (Chicago sports writer, 1893-1930's)