We were just recently at WDW with a group of 14.
I would strongly suggest not tryinng to keep a group of 20 together. Our largest group before this year was 9 and even that was not easy. It seems like people (even if you didn't have anyone with special needs) are on different schedules as far as who is hungry or tired or needs to use the bathroom. The few times that we did try to keep our group together, it was a lot of work to make sure everyone stayed with the group. We had one person leading the way (DH who knows the parks very well). I usually brought up the rear to make sure no one got lost (and people did all the time - someone would see something in a merchandise stall that they were looking at "just as we run by", but by the time they got done, the rest of the group was out of sight.
I'd suggest maybe going to the same park and keeping in touch with cell phones - it's really cool to turn a corner or go into a counter service restaurant and see people you know.
As far as lines go, as was already mentioned, the GAC is usually given out for a total of up to 6 (5 plus the person with a disability). If you haven't already, check out the part about GACs (Guest Assistance Cards) in the disABILITIES FAQs thread.
I don't think what Mickey'snewestfan suggested would work for very many attractions - it's worth asking, but many attraction don't have an alternate entrance and getting to the front of a regular line would not be that easy (in many, the front of the line is not accessible without going thru the whole line - so it's not like you can just wait to the side and hand the child over when the family gets to the front).
Also, if the child is using a stroller as a wheelchair - his stroller will be treated just like a wheelchair is for boarding. That means that for attractions where wheelchair users wait in line and then board at the exit so the wheelchair is waiting at the exit when you are finished with the ride. Most of those attractions have limited space at the exit, so the number of people who would be able to board with him would be limited - usually to 5 or less.
What DID work well for us and I would highly recommend is using Fastpass. You can send one member of your party to the Fastpass machine with all the park passes for your party. That way, you will all be able to ride together with a short (usually 10-15 minutes or less) wait. One thing to be careful of - they will not let you go in before the return time stamped on your Fastpass. So, check the times. We sometimes had 5 for one time and 5 for a time period starting 5 minutes later. They made us wait until the beginning time on the later Fastpasses.
Another thing that might be useful is having someone in your party look into Tour Guide Mike (he advertises on the Theme Parks Board). I have heard good things about his touring plans - since he knows what times things are normally busy, he can tell you what to see when to avoid long lines. One hint I can give is if you can get to a park soon after opening, you will usually have short waits, and this is the case even during busy times like Easter. We have been to MK and gone on pretty much everything in Fantasyland without waiting (some things more than once) by getting there warly in the morning.
And some attractions that are busy early in the day, like the Safari and Kali River Rapids at AK and Spaceship Earth in Epcot can be walked right on later in the afternoon.
