I am going to agree with bartleby1 that your wife will be fine on the majority of the rides that your 5 yr old will want to go on. I have neck, shoulder, and back issues and I was able to do even Everest and was fine and trust me it's rough. I have avoided roller coasters and rough rides for years because of this. This year I hit them all and came out with only a headache(due to the neck thing) after doing Big Thunder, Space Mountain, Goofy's barnstromer and the race cars, with my DD driving I might add, all in one day. I am not saying if she has serious problems she will be fine on everything, only that she'll be able to do a lot more then she thinks she will. Most of the rides are pretty smooth and slow paced. She would do great on Small World, Transit Authority, Spaceship Earth, Soarin, Finding Nemo, the Carousel, Peter Pan and so many others. Also I think you will be fine on most things. Goofy's barnstormer would be tough and the astro orbiter as it's 2 people crammed into one small space per orbiter, so definitely avoid that one. If you go on the Indy speedway you may have your knees in your face, but you would still get in. If you want another good resource for the rides and what they're like either buy or check out Birnbaum's Disney for Kids book. It gives a great breakdown on what rides are rough, scary, dark etc., and your son would probably enjoy looking at it and reading it with you as it's geared towards them. My kids love this book and it was great in helping to plan out the things we really wanted to do and what things we could skip because we weren't interested.. Have a good trip.
<waving hand wildly> I am 5'9" and at one time was 300 pounds (give or take a few). I rode everything with no problem. I thought I was going to have a problem on the rock n roll coaster at MGM, but all was well. 
