Make sure they have the correct shape. If not taught, kids will try to initiate the scooping action with a hollow shape in the belly/hip area, when that area needs to be flat with a hollow chest. I start with a horizontal cast and hold them in this position for a few seconds before I let them drop their shoulders back (at first you will be correcting the position, because they will most likely allow their hips to rise too high).
I like Gymsanity's idea of the seat circle vs. the toe circle to teach pushing, not pulling. If they pull the bar they will shift their wrists from the bicep and bend their arms. They will get nowhere fast doing that.
I'm not sure what drills you are actually using, but I like the standard approach of rotating them in a tight/straight back hip-like skill (no cast) while they push and shift their wrists. When those look good, I pull them off the bar to horizontal height. I don't let them do 3/4 or handstand so quickly.