The hard part is when you have a 14u team with some 8th graders waiting on the freshmen to complete their season. In Indiana, the better 12u teams will actually play up to 14u until the HS kids return. If a player is found to be participating, it can affect eligibility not only for that year, but the following year. There are also similar rules for each of the other HS sports regarding out of season participation. I guess you guys are actually lucky for your lack of a rule.
If your girls are practicing with the HS team 4-5 days a week it might be a good idea if you give them the weekend off. Nothing good can come from you going head to head with the HS coach and it sounds like you have one that cares which is real unusual.