Best to meet and get some kind of consensus on what people's expectations/priorities are for the trip. We just got back from a family cruise, and I'm not sure it met the expectations of my spouse's parents, who initiated and financed the trip.
I think they had dreams of family togetherness, shared experiences, seeing all the kids/grandkids interact, etc. Once we booked, our group split up for dinner -- not because anyone *really* needed to be early or late, but because people wanted things *their* way and didn't really think about the impact on the whole trip and the grandparents' dream.
That meant that we were split up for the evening, starting from 5 p.m. til about 10:15. It was hard to meet up afterward for things like the Pirate Party, etc. My kids were a little sad because they didn't get to see their cousins for some of the exciting dinner events and shows (they mostly saw them in the teeny-tiny pools, which were not our favorite part of the trip).
Not saying that dinner is the be-all end-all, or that everyone should have the same seating...but it is an easy way to connect each day, and it puts you on a similar schedule. Our daily activities didn't always co-incide (heck, sometimes I didn't even see my spouse til dinnertime, because we were off w/ different kids doing various things); so having that daily built-in interaction would have been nice. And I think everybody's kids would have survived at either seating.
If you decide not to share a dinner seating, try to arrange some other activity/ies ahead of time (we found it tough to coordinate once on board, esp. since we have lots of young kids), such as an excursion or two, a few sit-down lunches, grown-up nite at Palo, a meeting place on
Castaway Cay.
We ended up swapping a few family members at dinner one night w/ the group at the late seating (permissible per Guest Services, if you swap the same number from each group -- should try to let your servers know ahead).
Anyway, my bottom line -- you can't plan everyone's vacation for them, but if you don't try to arrange a few meet-ups (esp. at the beginning), your trip might whiz by without much real togetherness, as ours did. Esp. if you have small kids, plan ahead to share some time w/ the large group!
(It was still a fun trip, though!)
C