As an occupational therapist I will say that most if not all of the folks I have known in my life that use a wheelchair for mobility are used to this understandable curiosity of inexperienced children. Children ask disarmingly straightforward and honest questions. Your guidance as a parent is most importantly in teaching her to learn the social conventions of meeting this individual as a person first and foremost and when the time and place allow to approach and ask her questions. Be sure to sit in and see how your family member fields these questions and maybe you can help your daughter with her inquiry at the time.

Hopefully you will receive a good age appropriate book list from the bibliophiles, librarians and book experts here. But I wanted to respond and say "Bravo" for being the guiding parent you are.

As this isn't a direct answer to your question, I'm not sure if it will help, but, if your daughter plays with Playmobil, they have a couple of sets (the "specials"- small and cheap) featuring people with wheelchairs. I bought one for my young niece when my grandmother had a stroke and moved in with my parents because she was having hard time with the situation and I thought it might help her process the transition. It might be another way to familiarize your daughter with the concept of a wheelchair, and also a way to encourage and stimulate questions about the issue before you guys go on the trip. I know Top Ten Toys had a wheelchair Playmobil set a couple of months ago- they probably still do. However, my niece (who's almost four) is a very strange child who loves tiny things and doesn't put stuff in her mouth, so that might be a consideration, because Playmobil pieces can be pretty darn small.

Your daughter is probably way too young for this movie, but "Saved!" is awesome, and features Macaulay Culkin as a smart teen badass in a wheelchair (with a similarly badass girlfriend who, in the sweetest scene in the film, outfits her car so that he can drive it). It is one of the best, most positive and dynamic depictions I've seen of a teenage character in a wheelchair. The film also addresses (among other things) how obnoxiously smarmy it can be to showily "help" a disabled person.

My other thought (which might be more age appropriate) would be to look up some clips from Sesame Street on people with disabilities. A quick search on the Sesame Street website brought up quite a list of clips.