1) Don't be married to Manual. Stuff happens FAST at WDW, especially with small children, and you need to be able to whip the camera up and shoot, like, NOW! at any time. Metering and figuring exposure doesn't really go well with that.
2) With a 50D and a prime lens, you will be able to shoot lots of indoor stuff without flash, as long as you can rachet up the ISO a little.
3) Don't carry it in a bag. See #1 above for why.
4) Bring a longer zoom for Animal Kingdom. The animals are far enough away that 70mm is just not long enough.
5) Don't forget to rotate your memory cards; either download to a laptop every day, or switch cards every day and keep cards with pics on them in your room safe.
6) Florida's sunshine sometimes comes down in liquid form, so take along a
camera poncho.
7) Don't overload yourself. You've already got a lot of stuff to carry for the kids, you don't want to load yourself down to the point where you're exhausted by lunch time from carrying 100lbs of stuff. I carry no spare lenses, only a small pocket box with a couple of spare memory cards and batteries, and a lens cloth.
And have fun.