Thing have changed in the past year with Fantasmic and the dinner package.
Fantasmic used to be every night, but switched to only several selected nights a week as of January 2009. That so far seems to mean bigger crowds on the nights it does occur. In the past, they didn't do anything different for people with wheelchairs,
ECVs or GACs (Guest Assistance Cards) until you got to the stroller drop off area. At that point, those people were separated off and a CM showed them individually to seats. It was not that difficult in line because the line area is very wide and the people were walking in (so not waiting in a tight line) once the gates opened to start letting guests in.
With less shows, people have been reporting larger crowds waiting outside the gate before the show starts. As before, once they open the gates, the crowd starts moving and you are not in a tightly packed crowd.
For that situation, a stroller or wheelchair will help a lot to give your child safe 'safe space' in the crowd. If you are using a stroller, you can ask at Guest Relations to have it marked as a 'stroller used as a wheelchair'. That way you will be able to bring it into all lines and continue using it the same as a wheelchair. Some people find their child likes a blanket or sunshade pulled to better shield them from others and kind of create a 'safe cocoon' for them.
The other change is to Fantasmic dinner packages. There was a separate entrance, but that was discontinued. I'm not quite sure when or why, but there has been some major construction/renovation going on in shops on that side of the street. I don't know if the change in entrance is due to that or whether it will change back, but people currently have been reporting that they were told to go to the regular entrance up to one hour before the show. They were separated out once they entered, but if you get there after the time you are told to arrive, the seats that were saved for the dinner package may have already been released for other guests to sit in. (This was also the case before, but many people didn't realize it and thought the package gave reserved seats whenever they arrived. The change was that you need to arrive earlier).
Once you have seats, most people have not found the waiting inside the theater to be as bad as they expected. You can play gameboy, etc, eat or whatever in your seats.