She is getting to big for a stroller, so we are thinking wheel chair rental next trip.
Don't count out the WDW strollers yet. Some WDW CMs have posted that they have several sizes and the largest should fit up to a small 12 yr old. If you use a stroller, you can get a GAC that allows the stroller to be used as a wwheelchair.
If you decide to go with a wheelchair, I'd suggest you rent one from one of the outside rental places. These places rent children's wheelchairs (I don't have personal experience, but they do according to their website):
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
Colonial Medical
http://www.colonialmed.com/about_cms.html
I've read this child care equipment rental company rents special needs strollers (sort of like very large regular strollers).
http://www.all-about-kids.com/main.htm
There are a couple of reasons for renting outside:
1) WDW just rents adult size wheelchairs. While people do rent them for kids, the kids I've seen in them don't look very comfortable. The seat length is too long, the footrests are too low and the armrests are too high to be useful. They also have sling seats that are too wide for a child. OK for a few minutes to get from here to there, but not comfy for a day of touring.
2) You would have the wheelchair to use other places if you need it - like your resort or DD. You might not need it, but if it's really hot and humid when you go, you might be glad to have the option.
3) The WDW wheelchairs are quite heavy compared to some you can rent outside. On our last trip, we talked with a couple who had rented a "companion chair" from one of the outside places. It is meant to be pushed only - a regular wheelchair can be pushed by someone or the rider can drive it using the large wheels. The companion chair has just small wheels; more like a stroller. These people said they liked it better than a wheelchair because it was so light and easy to push and also was easy to fold up and put in their car trunk. Even if you rent a regular wheelchair from outside, you will have more choices of size and you can rent a lighter weight wheelchair.
4) A final thing; having a non-WDW wheelchair doesn't make you immune to people who think you are renting one to "cheat". My DD has an expensive custom manual wheelchair that is obviously much different than the rental ones, but we have gotten some "cheating" comments over the years. A different wheelchair does make it more obvious (to the CMs, if not to everyone else) that you did not just decide once you got into the parks to rent a wheelchair.