I would not suggest renting a wheelchair for a child, unless you can rent a pediatric wheelchair from home and bring it with you.
A 5 yr old would more comfortable in a stroller than a wheelchair.
The rental wheelchairs have sling seats and are very wide compared to a 5 yr old's body. She would have to stretch her arms out to reach the armrests, which are likely to be close to shoulder height. Because the seat is too deep for a 5 year old, she would either have to sit way forward in the seat so that her knees can bend (but then, she would have no back support). Or, if she sits back, her knees won't be able to bend and her legs will have to stick straight out - where the toe might get banged. The wheelchair is also quite heavy to push and higher to get into and out of than a stroller.
You will probably see lots of kids even older than your DD using the WDW strollers, so she won't be alone. If she can't walk in lines, you can get a Guest Assistance Card at guest Services in any of the parks that would allow you to bring the stroller into lines and have it treated like a wheelchair. You can follow the link in my signature to get to the disABILITIES Board if you have more questions.