Hi Linda!
The strollers available at the Walt Disney World parks are spacious, conveniently available at the parks, and easy to push, but I would consider how much use your granddaughter will need from it. The strollers are plastic and hard, and they do not recline, so it's something to keep in mind if you're granddaughter, for example, spends a lot of time in her stroller, or likes to nap in her stroller comfortably.
You can see some of the details on the rates of rentals
here. While I am unable to find a picture of the strollers available to rent on the Disney websites, a quick search for "strollers" on an unofficial site, allears.net, did turn up some photos.
I usually bring my own double stroller for my children, but in recent visits, Ive been turning more to the Disney strollers available for rent. Since I drive to and around Walt Disney World, it's of no inconvenience to me to be without a stroller out of the parks. (For example, at airports or resorts). Also, my children are becoming increasingly independent, and spend less time sitting in the stroller. On my most recent visit, when my youngest child did want to nap in the stroller, he was so tired that he made due with what we had in our changing bag and used a blanket and a change of clothes to "pad" the seat just a bit. He did just fine.
Hope this helps!