Welcome Lorraine ~
I am sorry to hear about your niece. An injury just before a trip to Disney is always a bummer. You will be happy to know that the Walt Disney World Resort has done a great job of making the theme parks and other resort areas as accessible as possible by providing many different
services for Guests with mobility disabilities.
I think your niece would benefit from
renting a wheelchair. That will be more comfortable for her than walking on crutches all day. Wheelchair rentals are available at all of the theme parks, water parks, and Disney Springs. Most of the attraction queues are able to accommodate Guests in wheelchairs but if not, speak with a Cast Member before entering a queue. You can save some money by purchasing a length of stay rental. This allows Guests to pre-pay the number of days that you will require a wheelchair for less per day when you rent for multiple days. You can do this at the first theme park during your visit. The wheelchair will have to be returned before you leave that theme park. When you arrive at the next theme park during your visit, you can show your receipt at the wheelchair rental location to get a wheelchair for that day.
Some attractions allow Guests to remain in their wheelchairs to board and other attractions require Guests to be mobile to board. You will find a list for each of the four theme parks on the link I provided you above. Some attractions like Peter Pan's Flight in the Magic Kingdom may be difficult for her because they cannot stop or slow down the moving sidewalk used for boarding.
If you have any concerns, please stop by Guest Relations and speak with a Cast Member. They will do their best to help assist your niece.
Wishing you minnie magical memories :-) Allison V.