Besides the hints already shared, if you look in post #3 of the disABILITIES FAQs thread, you will find some other hints in past threads that are linked there.
Many people find that a planning service like TourGuideMike is helpful. Even during the busiest times, different parks and different attractions have quiet times. For example, Kali River Rapids had a wait of longer than an hour last year when we were there in early afternoon during Spring break. By late afternoon, it was pretty much walk right on. That's what having good tour information can help you with.
I have been to WDW many times with my youngest DD who has ADD and have been twice with my nephew who has ADHD. One of the things we have found is that they had a much easier time waiting for things they wanted to see. 45 minutes in line was not too much problem for something my nephew wanted to see. 5 minutes was a long time for him for things he was not interested in. Some kids are more interested the more they know about an attraction. Some, it doesn't matter, but you would be asking for trouble if you try to see 2 or 3 in a row the child thinks is boring.