Some people choose to just find a spot and park along the parade route (be sure to turn power off and remove the key).
There are designated handicapped viewing areas marked on the park maps.
On the map linked below, it is marked with a red rectangle with a white P and a white wheelchair symbol. It will be in the same general area, but may be in a slightly different spot on the park maps you pick up when you are in the park.
http://www.wdwinfo.com/maps/mk.htm
There are handicapped viewing areas in each park and they are for night or daytime parades.
The areas will usually be roped off at least an hour before the parade starts. A CM will be monitoring the area to make sure only guests with disabilities and their parties are in the roped off area. They typically line up all the wheelchairs,
ECVs, etc very close together along the curb, with space for one or two of your group to stand behind you. The rest of the group would stand farther back.
If it is very busy, there may be several rows of wheelchairs/ECVs, one behind the other and all standing guests may need to be behind all the rows of mobility devices so everyone can see.
Depending on how busy it is, the handicapped viewing areas may fill up by an hour before the parade or may still have spots until right before the parade begins. Since it's not possible to know how busy it will be, it's better to arrive early (many people with disabilities choose to come when the parks are not likely to be busy, so general park crowds are not a good indication of how busy those spots will be).
Also, important to know that there are no handicapped viewing areas for the fireworks at MK.