I've been to both Disneyland and Seaworld several times. It's nice to have a wide angle lens, since you typically want some context, i.e. "here's my family in front of the castle" etc. I got by nicely with just the kit lens, but I can imagine a 10-20 would let you do some really cool creative shots. There really isn't much that requires a telephoto lens there, I brought a 100-300mm lens once, and the only thing it was good for was "spying" on people on the pirate ship
I wouldn't count on getting much time to spend on lens changes, the kids will be all over the place!
You should definitely bring the fast fifty, though, I assume you'll spend the entire day there, and it will be very useful towards the evening. I got this shot at f/2.8, 1/45s and ISO 400:
The next one is not exactly a great shot, but it was at f/1.4, 1/30s, ISO400, so I include it to demonstrate the lighting conditions. I don't like going above ISO400 on the K10D, but if you don't mind some noise, or are more confident in your hand-holding ability, I guess you could get away with the 40mm at f/2.8. Note: That's not my family in the front there, they were just in the way
For Seaworld, though, I would definitely bring a telephoto. You can get some awesome close-ups of the killer whales and dolphins. The 50-135 might come in handy: