You can usually figure out approximately where the handicapped seating areas will be a bit before the parades, when they start roping off things. The park maps tell approximately where the area is. So that should help you decide. I think most of the areas have at least a few benches. I know the one by the UK has benches as well as the one Lisa mentioned.
It doesn't sound like you'll be going during a busy time, but we have been at Epcot at times when only one person could accompany the person in a wheelchair into the special viewing area. There were a total of 5 of us, but the CM said it was too busy for her to let more than one person in with the wheelchair.
You might also want to consider just doing what you normally do, stake out a bench in the area of your choice and park her wheelchair next to the bench. This is actually what we do 95% of the time. DD is not patient enough to stay put in the wheelchair area until the parade starts. We've also occassionally had really bad spots in the handicapped viewing area - DD was parked right behind a tree. So, we find it easier just to find our own spot.