Personally I would cut a couple of days at Disney to perhaps 6? Assuming you are going for 14 days if you have 8 Disney + 3 universal + water park then it is pretty much every day taken care of. The parks will be very busy at Easter and you will appreciate a couple of days of not walking and queing.

With 15 of you going I'm guessing there are going to be some late nights etc and I'm sure a beach day would be welcomed by most of the party for some relaxation. If you are going for 3 weeks then 8 might work but I think you might have a lot of tired overwhelmed children (and somemtimes the adults) on your hands.

Just my personal opinion of course but I know our first trip we went for 16 days and tried to do everything thinking it was the holiday of a kifetime we'd only do once. I know we did Sea World as well and some went to Kennedy but we all came back needing a another holiday (and that was with just one 10 year old and 7 adults.