Need to rep it! Get them comfortable doing it. Other option is to just let them be lateHard to allow the flow (release) to happen when hitters are about to become vulnerable. Most kids freeze in anticipation.
Front foot needs to be on its way down at release. Slows the ball down. This check point is critical. What RH said about being rushed is correct. Ask your DDs if they feel rushed on good pitching. If they do, go learn to get the front foot up sooner.