Parallel bar swings. Do them every bar rotation at every level. Don't even worry about height just emphasize good shapes, and as the athletes develop strength and control they'll start swinging higher.
This is basically my silver-bullet, magic-wand approach to casts. It lets you crank out huge numbers of reps with no spotting and no bruised hips, builds the strength with basically no effort, and makes the mechanics of the skill feel intuitive. Start this from preteam, and I bet 90% of your kids will be casually casting to handstand by the time they reach level 5.