We took our 2 ds's in May when they were 7 & almost 3. I was worried about waiting in line too, especially since my oldest is pretty bad ADHD, but it ended up not being a problem. The best advice I can give you is to tour smart so you don't have to wait in line. We ended up avoiding almost all lines by using FastPass and early entry.
When we did wait, it wasn't a problem because there is always a lot going on around you. You are almost always moving and often there are TV's or props around to look at.
One idea I had read here is to take bubbles for them to play with, which we did, but we ended up never using them. I have also heard of taking Game Boys, or you could buy one of the Disney handheld games there, I think it is called Magical Moments or something, but I'm not sure. You could also play word games or something with them.
For the most part, waiting in line at WDW is much more fun than waiting in line at the post office or grocery store and my kids didn't seem to mind much at all. I seriously stressed out before our trip, and the best advice I have is to relax. Everything will work out great!
Kristy