Hey Samantha!
There is a card called a Guest Assistance Card, or a GAC, that can be issued from Guest Relations for your entire trip. The GAC will enable Cast Members in the Parks to help your family in any way that they can.
When you go to Guest Relations you will need to tell them what you think you'll need help with...like it will be too hard to stand for a long time in lines, or if she is taking steroids for the arthritis & it will be dangerous for her to stand in a line that is directly in the sun for too long...things like that.
One of our Super-Duper Disney Moms, Allison, has lots of experience bringing her little boy to Disney & using this GAC card so I encourage you to ask any questions directly to her on our board if you like! Just click on her name directly & she can tell you how it has helped her family.
My Mama is from Montgomery Alabama & has had Arthritis her whole life, so I unfortunately have a feel of what y'all are going through...give that brave little girl a hug for me! All of the Disney transportation is really smooth with no jolting & bumping so she won't have pain from that. You can rent wheelchairs or strollers at the entrance to every park. However the Disney strollers are hard plastic & might not be comfortable for her so I would bring blankets to cushion.
Y'all have a great time, full of magical Disney memories!