I replied on the other thread that you had posted this on.
Here's a link to the pictures of strollers
from allearsnet.com . WDW CMs have posted in the past that the strollers can hold up to a small 12 yr old. Some people choose to rent a double stroller for older/larger kids.
One other alternative is renting a wheelchair from the parks (or getting one lent to you if you are staying at one of the WDW resorts. Most of the kids I have seen in them don't look very comfortable.
The wheelchairs that are available are mostly adult size, with a 16-18 inch seat width. They are probably about 45 pounds empty, so pushing one takes a bit of exertion. The seat and back on the wheelchairs are sling style, similar to a director's chair (except the back is higher), so there is no support. Unless your child is adult sized, the armrests will likely be up between shoulder and neck height and the footrests will be too low for the feet to rest on them. For most kids, the seat will be too deep. That means if they sit all the way back to get back support, the 'bend' in the seat will not come at the same place as the 'bend' in the knees and the legs will be straight out in front. If the child sits forward enough that the legs can bend, they have no back support. That makes riding more tiring.
What you could do is try one out at the park rental area and see what will work best.
You could also look into renting either a smaller wheelchair or a special needs stroller from one of the off-site places. There is information about places to rent from in the disABILITIES FAQs thread near the top of this board.