I just noticed that this is your first trip! Congratulations! One piece of advice I have is....RENT the DOUBLE stroller! Some people bring their own (done that, other than MGM, disney's strollers are so much better) Less room for storage, but so easy to push. During our 2004 trip, our older boys (9 and 7 at the time) would take turns sitting in with the little one. Even if you think your older child would balk at sitting in the stroller, you'd be surprised when they get tired they don't seem to care.
Also, asking him to hop in when the crowd flow is bad helps keeping track of everyone so much easier.
We visit WDW every even year....this is the first time with our 4th child. Also bringing my little sis (15) and my niece (16) to "help" we'll see how that goes!
If you have any questions, please feel free to ask me, I've made a million mistakes...and i'm happy to prevent anyone else from making them.
Tracey