Just been away with our family, (5 children, spouses and 5 GC all under 4), we paid the deposit and totally for the children's rooms. They then paid the cost for their room. We get first choice of bedroom! Shopping costs were
split equally. We went out twice with the couples with no children, that was split equally, DH and I babysat for all 5 DGC one night, and all of us went out one night, the other three nights we all mucked in to create lovely in-villa meals. Alcohol was equally split, we all like a drink!, They all paid their airfares etc but I paid up front early to get cheaper costs. They paid us what they owed when they chose, and for some when we got to the villa. So we had no exchange rates to pay.
In the past when we have rented large houses in the UK, we have paid for the house, but all the food is then split between the 'children'. Each couple (except us) cooked their signature dish one night, we have a takeaway one night and a catered meal one night. We go out on our own one night. As do the other couples in pairs or alone, so not everyone is in on 6 nights. Works a treat! Whilst some could afford to pay towards the accommodation, some would find it a stretch. We just love having all of them together with plenty of bathrooms and usually two lounges/ games room. The grandchildren are getting to know one another well too, that is important for all of us. They then all go on another holiday together, as do we (several times). We are just enjoying it while we can.