My parents took each of their 5 grands on a 5th birthday trip. My twin boys got their trip in Dec and it was the only one the rest of the birthday child's parents/family came along. Mainly because my parents have gotten older and 2 is not easy as they found out even with our help. But the grands paid for the kids each time. Flights when needed, on-site hotel, AKL, Contemporary, or Wilderness Lodge, dining package and tickets. On this last trip, my wife and I paid for us and our daughter. It worked out amazing for the kids and my parents did an excellent job. It was something they wanted to do, and they never discussed how much it cost.
On the other hand, about 6 years ago we took another trip. It was something my grandmother wanted us to do after she passed. My family, my parents, my sister's family, and my uncle's family and his son's family. Grandma's will included us all going. It was a disaster because too many people wanted different things from the trip. Some wanted everyone together, some wanted all meals together, some wanted very little to do with anyone else. We had a 6 month old at the time, so 8 or 9 p.m. dining reservations and staying for fireworks didn't work for us, but others had little or no tolerance for the group's varied needs. We all ended up sniping at each other.
What I take away from all this? It all depends on who you go with and how the trip sets up.