I think I know what I'm going to do, and I don't expect it to be any issue with the rest of the party, just wondering what others would do.
A short break to do a specific thing, chosen by me. I'm going with DP and adult DS2. DP is not his Dad and we don't live together. ETA i.e no combined finances.
There is a double room with ensuite, and a twin with a further bathroom.
I will probably treat DS to his share, but he would pay if I ask him to, although he is in PT low paid work - longish story, but nothing that reflects badly on him, he's using the time to support elderly GPs with gardening and DIY.
The total cost is £300, which won't break any of us.
So split by person, by room, or just between me and DP and let DS have a freebie for the support he gives my parents (which in turn takes a load off me), or I cover the lot because it's my trip and I'm glad they want to come?