I have been researching cruises for the family for several weeks now, probably towards the end of the year. I get ships lined up and a few dates to choose from. Kids in the group are elementary school ages and a couple college students. Every time I suggest dates one or the other can't do it and the stalemate continues. Thought maybe a summer cruise, but prices are way up then. It looks like it isn't solveable (is that a word?)
Pick one that you want. Let everyone know that this is what you are doing. If they want to join you great, if not go without them.
You cannot please everyone, someone needs to be the leader as such. My guess is they will all go along with whatever you pick.