I really think a 3 day is plenty. Is it your first cruise for the family? If it is, then I definitely think 3 day is enough. You have to consider the fact that out of 6 of you, someone might get seasick (like Matthew did - which we were surprised about). If you go on a longer cruise and someone does have problems with seasickness it'll be harder.
We left on Thursday. Had Thursday evening on the ship, plenty of time to wander around. Went to dinner, Matthew got seasick as soon as we walked in to the restaurant, went back to the cabin. The next morning he felt fine and we were in Nassau. Went on an excursion - took a large catamaran out to the open water, got a tour of Nassau Harbor, went snorkeling in the open water, went back to the ship, went swimming in the pool. Went back to the cabin for a little downtime, watched a movie, could have gone to an afternoon movie in the theatre but decided to watch a movie in the cabin. Went to a live performance of Toy Story (fantastic!!!) and then we had a late dinner seating. That was the night that they had the pirate party on deck - but I had given Matthew some dramamine and he was ready to pass out. So, after dinner we went to the gift shop and then we went back to the cabin. He passed out and I watched a movie. They have a couple of different channels with continuous movies. The next morning we docked at
Castaway Cay. We went onto Castaway Cay and explored a little and then went snorkling there. We took a break for lunch (free) and then snorkled some more. I felt like we had been there all day and we checked the time and it wasn't even 2 yet. But we were both ready to head back to the ship. We went back, did a little swimming at the pools and ate again. Then we went back to the cabin for a while and then we went around the ship putting our DIS gifts in fish extenders. (Do you know about that???) Dinner that night was the best - Matthew wasn't sick or drugged

After dinner we went back to the cabin to pack - you have to have your luggage in the hall by 11 if you want it checked. Otherwise you have to carry it.
So, my opinion - 3 day is enough time!