The main things I can suggest are....
A) Plan a very light and casual schedule....the pace changes a great deal with small ones along. Build in nap times and don't stay out much later than the child's normal hours.
B) Make reservations for any meals (character or otherwise) that you want to do.
C) Be very careful about character interactions. Some kids love them and can't get enough...others are scared to death. My daughter would have nothing to do with them for years.
D) Use strollers, either your own or the parks. If you use park strollers, you don't have to worry about negotiating busses or monorails with them. You can also transfer a single rental between parks if you park hop.
E) Research parent swap on rides, or if you are travelling with a larger group, possibly trade off child watching to allow for you to do some other big people rides.
F) Be cautious on all rides. We don't think about it, but sound on many is loud, as are bright lights and rapid motions. The darkness is a problem for some. Even rides in Fantasyland have witches, scary people etc. Fireworks are also a problem for some for the same reason.
With all that said, have a great time, and take plenty of pictures as they probably won't remember any of it!