My small group hired a sitter for our nine kids. We paid per week, because sometimes the sitter couldn’t make it, or sometimes it was just a couple kids and we cancelled the sitter. We just rotated on who paid. So if another family paid last week, I’d offer to pay this week if I was there. It was an unspoken courtesy that each family offered to pay at least once a month. And that way, if you have a family that only shows up very occasionally, they won’t be expected to pay as frequently.
Eventually, once the kids got a bit older, like at least half of them hit elementary school age, we just put them in the playroom and let them duke it out amongst themselves. Actually, they played quite well together, and we were just one room over so could hear if things got out of hand, at which point one of the parents (usually the one to whom the screaming kid belonged) would head over to help the kids sort it out.