The strollers that you can rent at the WDW parks will hold "up to a small 12 yr old" according to what CMs have posted in the past and I have personally seen lots of big children using them.
Also since the OP is on a MAW trip, the WISH coordinator could get either a pediatric wheelchair or special needs stroller for them to use.
The WDW parks have a very few smaller wheelchairs, but most are either standard adult size (seat width of 16-18 inches) or wider ones. Depending on the size of the 8 year old, the standard adult size may work. But in many cases, the armrests will be somewhere between neck and chest height and the wheelchair seat will be too deep (if the child sits back all the way, the legs can't bend at the knees because the seat is too long; if the child sits forward so the knees can bend, they have no back support). For many people, a stroller just works out better.