It's wonderful that such an arrangement works for you: you have to be a pretty special family.
I've cruised with parties of 8 and 9 on the Fantasy in '14 and '15, both Western. For the first cruise, we had two connecting 4A cabins on deck 10. For the second, we had two concierge rooms, a 1-bedroom suite and a category V family stateroom.
* One slightly negative surprise was that some of the connecting staterooms verandahs on deck 10 can't have the divider on the verandah removed completely for structural reasons.
* A second thing, I was unprepared for, is how stressful it is to get seats together in the theater: at least half of our party would be oblivious to this issue and show up at the last minute every night, expecting us to have reserved seats for them (which is not allowed).
* Third, getting shared tables in the dining room is not a problem if you put in a request early. However, it worked much better for us when the children were placed at one end and the adults at the other.
* Fourth, on the first cruise, we found it difficult to find ways for everyone to sit quietly and talk together in between activities. It was always difficult to find tables next to each other where everyone could follow the same conversation. It wasn't that big a deal since people would split up and do things in smaller groups, but it is the biggest thing that I wish that I had been more prepared for. We were 3 generations traveling together and the grandparents had some mobility issues, so in your case it probably won't be as important. Still, that was what convinced me to upgrade to concierge for our second cruise. It worked perfectly for our family as we ended up spending most of the time together, just enjoying each others' company in several quiet locations that we did not have access to on the first cruise. Concierge is certainly not necessary: I am mainly pointing it out because I wish I had planned for deliberately splitting up our party into smaller groups instead of always having to struggle to find two nearby tables at Cabanas, for example, where we would not really be together anyway. In my own experience, the best way for ~10 people to do things together (short of concierge) is really the port adventures, including just about anywhere on
Castaway Cay, so I'd try to pick some that everyone can enjoy together and then put an emphasis on keeping the group together at those times.
Edit:
* Fifth: count and photograph your luggage. We brought 17 bags on our cruise (after leaving another 8 in long term storage at our Disney resort) and, when we picked the larger ones up in the terminal during debarkation, one was missing. All the staff were eager to rush us through customs quickly and we had to insist that our bag was gone. It turned out that another family had taken it by mistake, but we would not have gotten to fill out insurance paperwork or get the same help from
DCL if we had just gone with the flow.
* Sixth: many of us tend to be on the OCD side of planning our trips. I hardly planned at all for our first cruise, but, although it was great, I saw later that my family, none of whom care at all about planning, would have had an even better time if I had been better prepared. So, after that, I made it my mission to learn everything I could. However, when we actually are on vacation, I force myself to step back and let others improvise on "the small stuff", even when I cringe when, for example, someone suddenly wants to spend an hour touring a tequila outlet in Cozumel which means that we will miss snorkeling that others had been looking forward to. I just step back and let others decide in cases like that, otherwise it would create tensions in a large group. That means that my family trusts me when I do have to step in and say no: for example when a cast member incorrectly told us that our customs forms were filled out the wrong way right before we had to go through immigration (after I had spent half an hour the evening before carefully filling them out). Unexpected complications like that can create tensions in a large group when several people want to respond in different ways under time pressure. Some complications can be predicted, for example a scheduling conflict when an Anna and Elsa greeting happens at the same time as most people are leaving the ship on Castaway Cay day. I try to anticipate those and talk with the family well in advance about what to do, seeking their input.
Anyway, these are just some observations that I wish I had read before my own cruises. Realistically, though, each family is different and you'll end up having an amazing time no matter what.
