Love, love, love Seaworld for younger kids.
I usually find the best deals are on the Seaworld website.
Higly recommend that you take more than one day for Seaworld as there are tons of shows that kids like, plus all the animal exibits. Don't forget Shamu's Happy Harbor with rides for the little ones. Seaworld also has lots of places where kids can get wet (so take a change of clothes or two).
Weather in December should be nice, but it could get cold if a cold spell creeps down from the north. For that reason I wouldn't have my heart set on a waterpark, but plan one in with a back up (just in case the weather is too cool). All of the water parks have special areas for the younger kids (my DD loved typhoon lagoon when she was 2 - we even took her swimming with the sharks!).
For a single Disney Day I would go to Mickey's Merry Christmas Party - you can get in from 4pm and its not as pricey as a 1 day Disney pass.
Check to see if it is snowing in Celebration (In december it usually does in the evenings, on the hour, but perhaps only at weekends)
A nice place for kids is Green Meadow's Petting Zoo. December would be a great time to visit as the weather should not be too hot.
Universal have nice toddler areas, In UStudios, they have a Barney show which (unfortunately) my DD loved - I think I know the words off by heart (but it is fun for the kids!!), they have an animal show, a great playgound and more, In IOA they have a Dr Seuss land with rides, a large dinosaur playground and more.
HTH
--Kay
