I just saw this drill on the parallel bars the other day where you have one bar set up higher than the other, pad the higher bar and have them cast on the lower bar, facing out, with the goal of their legs touching the padded higher bar.
I might not be explaining this very well, sorry. It's late.
This drill also assumes they are already casting with good form.
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.