Hi Katie,
What a wonderful aunt you are to plan ahead for your niece! Be sure to stop by Guest Relations & pick up a Guest Assistance Card for her. If you are not familiar w/ it, let me know & I'll be happy to share some details w/ you.
If you are thinking of Disney stroller v. wheelchair rental at Disney, I'd vote for the w/c. I know that it will be more $ but the strollers are made of hard plastic & will not be very comfortable for her. If you purchase the
Length of Stay option for the w/c or stroller, it will save you a little $.
Strollers & w/c must remain in the park from which they were rented so if you park hop during a day, you will be required to turn them in b/f you leave that park & then show your receipt at the next park to get another one - subject to availability.
If she doesn't have a w/c or stroller of her own that she could bring, you could check into renting a special needs stroller from a vendor in the Orlando area.
www.orlandostrollerrentals.com offers free delivery & p/u to Disney & other area resorts. They'll have your stroller waiting at Bell Services when you arrive & will pick it up there after you depart. Renting from a local vendor guarantees you'll have a stroller & allows you to use it throughout the WDW resort.
I recently assisted another Guest regarding using strollers in line for children w/ special needs - click
here to see that answer - some of the info may be of assistance to you as well.
My space is limited so please let me know if you have further questions or need more details. I'm happy to assist you in making this an awesome trip for your family :-) Allison