I absolutely agree with Vanryan about the strollers. Either buy or rent 2 (you don't want a double stroller because it is likely that you will be splitting up sometimes).
We went last Oct. with our DS6 and our DD3 (one month short of being 4). I actually bought a used, heavy duty stroller for my son, which I will soon sell and get most of my money back. When he first realized the stroller was for him, he said that there was no way he was going to use it. Of course, he happily used it everyday (not constantly, but when we were going across the park). Having a large basket to put stuff into was great, too (my DD's stroller was a bit smaller).
Make sure you have the kids wear comfy shoes with socks, and bring a pair of sandals for the water rides. Also, my DD did not want to get wet on the Kali River Rapids Ride, so I let her wear her poncho (purchased ahead of time at Big Lots) and she was fine.
Bring some small snacks for when you are standing in line -- we have found that Tic Tacs work great -- very few calories but the kids think they are a big treat -- and they don't take up a lot of space.
Also-- the autograph books are a good idea, but I used something else. I bought 2 t-shirts from the
Disney Store and with a fabric pen (the kind that raises up when you write) put there name on each plus the dates of our trip. I then put it in a plastic bag (in case it rained), along with a Sharpie, and kept it in the stroller. The characters were happy to sign the shirts and now the kids have a wearable souvenir for about $7 each (shirts in the park are about $18 each). Obviously, light colored shirts work best.
Lastly, we love the Chef Mickey's character breakfast -- we get a PS for the last seating (around 11:15 am) and treat it as a lunch, just munching on pop tarts or granola bars for our breakfast. The Chef Mickey's dinner is also good. Plan on going on a day you will be at the MK, because the restaurant is in the Contemporary Resort, which is only a short monorail ride away.
If you have any more questions, just ask!
Have Fun!!!