Plat around with them standing on a block that puts them at a height where their hips are just above the bar with their shoulders open to 45 degrees (or more) and a slight lean forward.
Now have them hop/jump into a cast position and clear hip around to the block. Have them start with the block a little close at first. but not so close they'll hit their head, and gradually move it away from the bar until you get the results you want.
Once they have that, have them land softly on the block, like one that makes no noise or doesn't rock/shake the block. This will promote the idea of proper hand/wrist shifting to get their hands to the top of the bar in time to support their weight at the peak of the circle, and there's almost no chance to land soft if they lose the correct shape on the way up.
If you have them do sets of five where they circle, land and plie', and immediately jump into the next one they'll tire a bit by the 4th, and will have to resort to excellent technique to make the fifth....... or something like that.
Maybe start this out as a fun game/experiment, and see how it evolves for a few weeks before tweeking it to get it the way you want.