to OP remember that at MK even when you drive you must take either the ferry or the monorail to the front gate as you can not walk from parking lot to gate
Since you've rented a car, you can do a test with her. If someone else can drive the car, you could take her back via the Disney bus and see if she gets ill. Try it with a short trip first... i.e. if you're at an All-Star do it on the way back from AK as that's the closest park to the hotel.
If the OP decides not to rent a car the "easy" solution is that if there are not seats near the front of the bus because you are in the end of the waiting line to just wait for the next one.
I have one that is motion sick easily. She has a scopodomine patch to make our vacation uneventful. At the very least the sea bands or Bonine should be used. It's terrible being in the heat, crowds and motion on rides to end up ill feeling or throwing up. Been there and it isn't pleasant for the kiddo or parents.