Besides what has already been posted:
There are a few handicapped parade and Illuminations viewing areas with benches, but those are at the back of the viewing area. So, if you sit there, you will be behind all the guests with wheelchairs.
So that those in wheelchairs don’t have someone standing in front of them, CMs line up all the wheelchairs across the front of the area. Usually the wheelchairs are tightly packed, nearly touching, so they can get as many people as possible into the front row. The standing members of the party will be standing behind the wheelchairs from that party.
The areas tend to fill up quickly and early and sometimes some areas are in demand enough that there are 2 rows of wheelchairs, one behind the other. We have already been at WDW at not very busy times when some of the smaller viewing areas were already totally filled more than an hour before the show.
Specifically for Illuminations, you could be better off just finding a spot. The show will be on/above the World Showcase Lagoon, which has a fence all along it. It probably will be easier for you to just find an area along the fence to stand or sit on the ground to watch Illuminations.
and, as the others mentioned, there are no handicapped viewing areas for fireworks at MK.