My 3 generation family trip was much like
@PSUGuy describes, though I was some kind of "elected dictator" and I didn't foot the bill!
We had a sit-down meal all together once per day and went to the same parks (we didn't do hoppers) and I tried to tell everyone "here's what I'm doing. It may not best fit your sub-group, so feel free to split up and we'll reassemble" but we rarely did split. Pretty much when there was a split between thrill ride or not, we split just for that ride and rejoined quickly. One day my group didn't rope drop but everyone else did and we met up before noon, remaining together after. After dinner or evening fireworks, we split off to our rooms, so some evenings, that meant a bit more personal time (rest, pool, tv) from 8:30 or so on, but some eves that meant straight to crashing in bed.
I would set a minimum expectation of togetherness - perhaps one meal per day, perhaps rope drop through lunch, etc. then let folks do as they like, which may still include a lot of voluntary togetherness!
If you plan to split, make sure you "assign" each couple or sub-group some time with you, so you don't find at the end of the trip that you completely missed out on together time with one person.
I think it's best to plan some private time daily and be pleasantly surprised in everyone wants to stick together, vs planning too much group time and finding someone going nuts, not sure whether to rock the boat and defect!