Depending on the message on the GAC, you may be able to use the handicapped viewing areas.
But, as the other posters mentioned, those are often very crowded and you do need to get there early to get a spot (plus the person with a disability needs to stay in the handicapped viewing area).
Normally, the wheelchairs and
ECVs are parked, nearly touching each other, across the front of the viewing area. There is space for the rest of the party to stand behind the wheelchair.
Some of the handicapped viewing areas are not very deep (the one in Frontierland), so there is space only for one wheelchair and the people standing right by it in the depth.
Some others are very deep and if your child would not tolerate being at the front of the space, your family could hang back with him at the back of the space. You would be far enough back in many cases to still see. This works at the MK parade viewing spot in front of the castle and also some of the viewing areas at AK for the afternoon parade.
I agree with the other posters that you may have much better luck just finding a spot outside of the handicapped viewing area and arranging the rest of your party to act as 'buffers' for your child.