Agree with @Jard.the.gymnast, we always teach a double back hip circle first. Once they are confident with that and they can do it nice and fast with good form, we move onto placing a wedge mat in front of the bar that they undershoot over (teaches them to get their hips up). Then it's lots and lots (and lots) of repetition
Mine were doing really well with these when I was doing 2 BHC in a row and a couple of other things for them. Now that I moved them on, they are getting bad habits again. Such as throwing the head back, dropping the bar to their knees/shins or just dumping off. I'm a little frustrated.
I also agree with the above! Teach double back hip circle first, otherwise it’s really for kids to understand the concept of the underswing keeping the hips close to the bar. It is also great prep for clear hip circles.
I like to teach double BHC’s on the low strap bar, with a bar pad under their hips. They have a bit more momentum and it’s also a good way to prepare them for learning clear hips in straps.
I also do spotted slow-motion drops to candlestick - freeze - spot the undershoot. Then add in the BHC and work on speeding things up. Lots of girls I've worked with tend to throw their head back so this way they can really focus on keeping it in (with added verbal corrections as well).