Try this........ Stand behind them while (staddling their back leg) they're in the split position, and lift them up slowly (about 1 foot) while they press both feet into the floor as if they were trying to slide them toward each other. Then lower them down until they're holding their split position, and then lift up again....repeat 10 repetitions.
Once done.....
Let them slide as low as they can, into their split, without any pain. I'll bet you they get lower than they have when somebody pushes them down.
Do it every other day with half the kids, and the alternate days fo the other half of the kids. Teach them how to lift a partner and they can pair up and do it every day...... left, right, and gently on straddle splits.