Most of the lines are shaded/covered since they have to deal not only with Florida sun, but also with the Florida rain. Even the lines outside have things like fans to help move the air and some are even air conditioned.
There is information about Guest Assistance Cards (GAC) in the disABILITIES FAQs thread near the top of this board. The GAC information is in post #6.
As OneLittleSpark mentioned, the GAC might not be all that helpful since most of the lines are covered. You should know that almost all the lines at Epcot are inside, so that is one park that will be less of a concern.
Different parks are busy on different days and times. For example, even during busy times like Spring break, you can get into most things at MK with a very short (less than 10 minutes) wait if you go at park opening.
If you go to AK in the middle of the day, it is quite busy, but by 3pm most of the rides are walk in.
Another thing that will help you for some attractions is Fastpass. After inserting your park passes into the Fastpass machine, a Fastpass ticket pops out with a return time. When you return, your wait will be less than 10 minutes. To avoid having your whole party moving around so much, you can send one person to the Fastpass machine with all your park passes to get Fastpasses for the whole group.