Teach them how to warmup and stretch. Why they do what they do in warmups and stretching. I have to remind my son when he does lessons to explain why not just how. I would have them start from basics with everyone. Takes time but don't assume. I have high school pitchers that no one has ever taught how to hold a ball. Walk through steps for windup and stretch. Then break down into groups according to skill level. Make sure they put on the form for the parents what age group they play and how far mound is from home plate. i would also strongly recommend them sign a waiver in case. won't keep them from getting sued but will help.