My DD usually has at least one seizure every trip to WDW. Things to watch out for are dehydration and changes to her schedule since they are things that that can lower the seizure threshold (how easy it is to trigger a seizure).
Whether or not there is a quick way out of either the line or the attraction depends on which attraction you are in.
The shows (theater type things, such as Voyage of the Little Mermaid or Muppets 3D) have marked exits, so leaving during the show is easy - just look for the exit signs. Those shows in general have a "preshow" area where they sort of gather a group of people to "load" into the theater in preparation for the next show. Those areas have some way to exit in case of emergency and there is usually a CM around somewhere who can help you. Occasionally, you might have to wait in the preshow area until the show "loads". If that happens, just continue into the theater and cross all the way across the theater. The exit is usually directly across from where you entered.
For the rides, you will not be able to exit during the ride until you finish and get to the regular exit point. If you are in line, you may be able to exit, but not necessarily right away...sometimes you have to continue forward until there is a gate or a chain that can be opened. It used to be that the lines were pretty narrow (to keep people in single file lines). When they started making the queues wheelchair accessible, they made them at least 3 feet wide, which means there should be room to get back out of the line the way you came in.
If they need to rest afterwards, you can go to First Aid in any of the parks - the CMs there are very nice. It is dimly lit, quiet and peaceful and they have individual cubicles or rooms with a curtain for a door and a cot to lie down on. First Aid stations are marked on the park maps.
If we can find a spot in the shade, we often just park DD's wheelchair. Hers doesn't recline, but if we flip up her antitip bars and put up her headrest, I can sit on a bench with the wheelchair leaning against my lap. Her backpack makes a good pillow/cushion.
