I have twins - boy.girl - 6 yr olds. The first time we took them to Disney was when they were 16 months old. We had a blast. I ditto all of the advice of the previous posters. My son is not as aggressive as his sister, so we tour with differently with him. She is a daredevil and will do anything. He, on the other hand, likes the slower pace of the world.
We also took them when they were not quite 4, and again this past April.
I have to say, I remember the first time we went was in February. It was perfect weather. They were very comfortable napping in their strollers - make sure you get one with an umbrella. This allowed my husband and I to go on lots of rides - lots of times - when they were napping.
The first time, we flew from Providence, Rhode Island. On the way down they cried, most likely due to their ears. On the way back, they slept the entire trip home
These last two times, I made sure they took a decongestant before their flights. This past April, we used the decongestant and those new airplane ear plugs (forgot the name). Anyway, the flight did not bother them at all. We had a great flight.
Have fun. Take lots of pictures. I cannot tell yoyu how many times we take out the photo albums and reminisce.