Not to worry!
Make A Wish guests get a special pass that allows them to have fast ride access with their party, so don't worry about that. You will be well taken care of.
Most of the Make A Wish guests stay in Give Kids the World Village
(click here for a link) and are totally pampered during their stay. There are a number of fairly frequent posters to this board who have had MAW trips, stayed there and can tell you all about it.
There are some differences in the way things happen in Disneyland as opposed to WDW. DL was built way before anyone thought of people in wheelchairs going to an amusement park. As a result, most of the rides had to be retrofitted, some lines can't fit wheelchairs and wheelchair users have to go in the exit of many rides in order to board. Some of the rides at MK at WDW are built the same way. When we first started coming to WDW 15 years ago, wheelchair users had to go in the exit because that was the only way to get to the place where they could board. As rides are being changed for fastpass, the waiting lines are being adjusted so wheelchair users can wait in the same lines with everyone else (Mainstream Access). Some of the rides/attractions at Epcot were built with wheelchair users in mind and almost all of the Studio and AK were built with mainstream access.
For some people (like Wheelsie, for example), what they need is a line that will allow them to bring their wheelchair with them where ever the line goes (plus a way to board the ride).
Fastpass also helps with the wait. When you get a fastpass, a return time is printed on it. When you come back at your return time, you may have only a 5 to 15 minute wait. That helps a lot. The GAC that Wheelsie mentioned also can help. It is not meant to give immediate access to the ride, but will help you get what you need to enjoy the park. It might be a waiting place out of the sun or an alternative waiting place.
If I did this right, it should be a link to teri's FAQs about GACs. If it doesn't work, go to one of her posts and click the link by her signature.