If you think about it, many times the kids are told to kick their foot harder or get their foot higher, but don't get just how ripe those corrections are for misunderstandings. The thought and correction process works better when they are told to kick their leg or get their leg higher, because they are then at least thinking in terms of their entire leg.
A quick corrective tack that's worked for me is to tell them which muscle is going to feel the effort when the leg is lifted behind. That and telling them to stop focusing on the height of their foot, and instead put that energy into lifting their knee up behind them while keeping their quad muscles tight.
So the new goal for the day is to stop worrying about the degree of split, but to concentrate on the knee kicking up behind them and to feel the quads staying tight. I don't care what your results are other than to see what I want being your top priority. Do that, and we'll both have something to smile about!
And...
The back leg is prone to bending as a result of muscles being too weak to lift that leg up behind them. That befuddles coaches who have gymnast capable of over splits stand next to the beam and kick their leg up behind them while using the beam as a ballet bar. These coaches quickly add 2+2 and conclude that over splits plus horizontal back kicks equals a 180 split leap that's perfectly level from front to back foot.
The problem is that back kicks next to the beam are solidly anchored to the floor via the support leg, and forget the advantages a solid base provides. They then tell their kids their back legs are always low because they just don't try hard enough.
That may work if the coach is sitting in a big red vehicle from which authority can be projected (hey it could just as easily be a fire truck ;-0), but I think the better choice is to teach them a technique that makes so much sense that it works, something like this:
Our bodies are made to allow our legs to lift up in front of us, while our back legs..... not so much. So if you want a split that is equal at 150, 165, or 180 degrees, wouldn't make sense to put the bulk of your split effort into the back leg. You know.... back leg x 3= front leg..... Sorta like if the back leg is three times harder to lift behind you than the front leg, then should all your effort go into kicking/lifting the back leg up and let the front leg become an easily adjusted after thought.
It's just a bit more complicated than that, so a step is added where the front leg gets some swing and then the back leg goes hard, which gives it the energy to get the best height possible and to create a reaction that propels the front leg through the rest of it's lift. It works almost every time for kids who are not afraid to take off and commit to the jump or leap.
