As far as where to sit for the fireworks. There is a handicap spot near the castle where I sat with my family and we were able to view the parade and the fireworks. All they had to do was spin me around in my wheelchair so I could view the castle.
Sometimes that works and sometimes it is too crowded and it won't work.
The designated handicapped viewing area for parades is right in front of the castle, facing a bit toward Tomorowland. Near the end of the parade, the CMs take down the ropes that mark that area. After the ropes are gone, other guests can get into the area.
If it is not too busy, just spinning around may work. When we have been at WDW when it was busy, the area was so full of people that our DD had people standing literally at her feet and she was not comfortable staying there for the fireworks. She had kind of a wall of people and could only see straight up.
You can click on the link in my signature to get to the disABILITIES FAQs thread that was already mentioned.
Post #6 of that thread is all about Guest Assistance Cards. In post #3, there are some other things that may be helpful to you and there are several threads on the 2nd page of that thread about accessibility and which attractions have wheelchair accessible ride cars.
Although people may think they are 'disability cards', GACs are actually information cards to let CMs know what sorts of assistance guests need because of their disability, as was already mentioned.
If all the person with a wheelchair needs is an accessible line/boarding area, they do not need a GAC. The CM can see the wheelchair and know what you need.
At AK and the Studio, the whole park was constructed with "Mainstream Lines" so in most cases, there are no 'handicapped lines.' These are lines that are wheelchair accessible so that guests with wheelchairs are able to wait in the same line with everyone else. There are a few attractions at those parks where part of the line is not accessible - for example, at Kilimanjaro Safari at AK, guests with wheelchairs wait in the same line with everyone else until they just past the stroller drop off point. Right after that, there is a wheelchair pull off point to the right side. That allows guests with wheelchairs and
ECVs to leave their mobility equipment at the boarding area and that tram will come back to the same point to unload. That would not be possible at the regular boarding spot, since the tram loads in one place and unloads at another.
Loading at a 'special place' does not mean the person will board any faster, especially if they have to wait for a wheelchair accessible ride car. For the Safari, we have waited as long as 40 minutes extra once we got to the handicapped boarding area.
At Jungle Cruise recently, we had to wait for the special boat and waited for 3 boats longer than we would have waited if we could just walk in line (the woman ahead of us when we entered the line had a pretty distinctive dress on, so it was easy to see when she boarded).
Most of the rides with moving walkways can be slowed or stopped for boarding. AS KPeveler mentiones, the exceptions are Peter Pan, which can't be stopped or slowed and Tomorrowland Transit Authority (People Mover) which has a steep moving ramp to get up to the boarding area and a moving walkway to board and exit. With a wheelchair, you don't need a GAC to slow it; just tell the CM what you need.
All shows have spots where a guest in a wheelchair can remain in their wheelchair for the show. In movies and many shows, the wheelchair seating is in the back row.
A few shows have the wheelchair seating in the front row (Festival of the Lion King at AK; Laugh Floor and Tiki Room at MK).
A few have some seating in the front row, although the majority is in the last row (Indiana Jones Stunt Show and Beauty and the Beast at the Studio, Nemo at AK).
There are handicapped viewing areas for parades at each park. The spots are marked on the park maps. Two things to be aware of:
1) Check out the locations ahead of time. The areas for the day parades may be right in the sun, so you may actually do better just getting a spot by the curb in a shaded location.
2) Many of the viewing spots fill up quickly and are very crowded - wheelchairs parked almost touching across the front of the viewing area with the rest of the party standing behind. Sometimes, when it is very busy, they do limit the number of people allowed to accompany the person in a wheelchair to 1 or 2.
3) There are viewing spots for Illuminations at Epcot (again, marked on the park maps), but there are no specific spots for Wishes at MK or for the castle forecourt shows at MK.
A couple of things to think about for room reservations.
1) Some people with special needs prefer not to use a
travel agent because they would rather explain their needs directly than have to explain to a travel agent and hope the travel agent understood and was able to communicate that to Disney.
2) Many people think that you need a handicapped accessible room with a wheelchair, but with a small child, you may be better off with just a regular room on the first floor or near an elevator.
There are 2 types of handicapped accessible rooms and neither is any bigger than a 'regular' room. The first type has grab bars in the bathroom and may have a raised seat toilet, but is otherwise the same as a 'non-accessible' room.
The second type has a roll in shower that a shower chair can be rolled right into. It also will have a raised seat toilet. In order to make space for the roll in shower, the bathroom has to be larger. The room itself is the same size as 'non-accessible' rooms, so to get room for the bathroom, the bedroom area is smaller.
There is more information about rooms, including pictures in post #7 of the disABILITIES FAQs thread.
3) Raised seat toilets and 6 year olds may not be a good combination - we found that out the hard way and after that did not reserve an accessible room until our DD was over 5 feet tall.