F.H.
Well, this would be easy if it was just drop and go, you could just say in the invite that the cost for the kid is covered, but if parent is going to stay, they need to pay themselves. BUT you are asking/needing parents to stay to supervise, so in essence, you are going to be asking them to pay for themselves anyway. So you would need to word it accordingly if you need parents to stay. Otherwise, I would try to get grandparents, aunts, uncles, etc, who may already be in attendance to be the supervisors of the kids. Also, you tell them on the invite that you need a firm headcount by Wed at 6pm so please reply if you are planning on coming by then. I hope it works out for you. Good luck!