Was there a relationship between the long 12 yrs of the cruise and the short 12 yrs of the twins (like maybe 9 months)?
I just made another post about the kids programs. They are great. When on the ship, I always made sure that we ate diner together. While we gave them extra freedoms, we made sure that they were under control and respectful. My son was a handful at times.
We always included them in the decision of shore excursions, sometimes pushing them toward a tour we would enjoy. That cuts down on complaints. Might want to buy some snorkeling equipment at home. Not much more than renting it, and it beats using equipment someone else just was slobering on.
Finally, Mariner leaves from Pt. Canaveral, conveniently located by KSC NASA, and Orlando's attractions. You might want to rent a car and stick around a few days.
Enjoy your trip.