With kids of those ages, and with special needs, I think your #1 consideration should be personal space for all those concerned. For that reason I would go with a villa or a/couple of condos/timeshare units over two hotel rooms. The logistics of sleeping are a potential nightmare, you don't say what sex the 15 and 7 Y old are, but if you go hotel rooms then either adults are going to have to sleep in the same bed as their kids or between them the 15,13 y old special needs and the 7 y old are going to have to share 2 beds between them some how. IMHO it has pretty sizable potential for unhappy campers. Getting a 5 bedroom villa would allow both sets of adults to have a bedroom ( 1 Y old I'd guess sharing with it's parents, but you could move it about if mom and dad got a few too many sleepless nights) and leave a bedroom each for the kids ( maybe & Y old and 1 Y old share). I'd be certain the 15 Y old will appreciate their privacy.
You'd have to check the airline/
Travel agent for details on the wheelchair question, even if it had to go in with the luggage there would be plenty of wheelchairs at the airport for you to use there once your own had been loaded, although it's always a good idea to put the request in EARLY and get a written reply (TAKE PRINTED COPY TO THE AIRPORT) to your request.
As far as DC goes, they have special wheelchairs for special needs guest with extra large tyres on them to deal with having to be driven over the sandy beaches. Again if you want to go the DC, make sure you get your request in early for a wheelchair to use, get a written/emailed reply, print it out and take it with you. DC get a lot of special needs kids and are very well versed in looking after them and from what I saw did a wonderful job, but it's always a good idea to have a written confirmation in your posession, just in case something gets lost in cyberland.
I would agree with Hilary, if money were no object, I would try to stay on site. However with the make up of your group, it's going to be a very expensive option. Two rooms wouldn't really do the job, more rooms (four) 2x2 connecting rooms would do the job, but would be expensive. A DVC unit (two bedroom) would still need the kids to share 2 beds between 3 of them and IMHO with a 13 Y old special needs in the equation, even if all the kids get on well, it's a potential problem that can be avoided. Two cabins at Fort Wilderness would give you the required space, but with a special needs kid, getting to and from the units, might pose a problem. Whichever way I look at the on site options I see no easy, obvious answer. Granted the park access is helpful for being on site, but IMHO you have to ask yourself if that hour you save having easier access to the parks is worth the risk of everyone having a difficult time sleeping. If you're short of sleep, even 20 minutes on the bus is going to be a chore, if you're well rested and relaxed the 40 minutes drive in (less if you select a well located villa) may be the better option.
With a special needs kid you're going to have to plan in plenty of "relaxation time" for her, with 4 adults that shouldn't be a problem. One set of adults can be with her and the 1 Y old if they need a break, while the other set of adults can take the 15 and 7 y old onto things not possible for the others. If you think that a whole day in the park would be too much for her and the baby it might be a good idea to consider two cars, so the adults could take it in turns to leave the park early in order to take her and the baby home ahead of the others. IMHO a second car would be a cheaper option than having to get 2 extra hotel rooms on Disney. It would also allow the pairs of adults to occassionally have a quiet evening out on their own.
Broken down into it's most basic form (money)
1) a villa should set you back in the region of $120-150 a night for a 5 bedroom in a convenient area.
2) 4 rooms at moderates is going to be about $120 per room per night ( i.e. $450-500)
3) a DVC 2 bedroom unit would be about $350-450 a night),
4) two cabins at Fort Wilderness about $150-180 per night per room.
5) Two off site time share units would give you more "hotel like" facilities, but is still going to be in the area of $300 a night.
6)Something like the Holiday Inn Kids suites would be an option, they offer a queen bed for the adults and a seperate "kids area" in the room with two bunk beds (some rooms have two bunk beds and another small single "cot" which would do for a smaller kid up to I'd guess age 4/5). You could get with two of these style rooms (cost about $130 a night per room) and still have hotel facilities (pool etc and I think they have a shuttle to the parks if some of your group needed to leave the park early and you only had one car) although you're losing the convenience of Disney's transportation.
Without a personal understanding to the group it's impossible to say which would be the best choice for you, I tend to take the rather simplistic look that one is likely to spend 8/9 hours a day in the sleeping areas and maybe 1-2 hours travelling, therefore the sleeping arrangements are the higher priority.
Oh another factor may be that brekkie with a 1 Y old and a special needs may be much less traumatic in the comforts and space of your own kitchen/home than they are if you have to eat out in restaurants all the time. It may seem a small consideration, but having your own kitchen,dining room and space may be something that allows you the option of some meals staying at home cooking or getting a take out and not having to deal with what can at times be a stressful occassion. If things go well, then you don't need to use the option, but if baby or special needs get tired/stressed out/willful or unco-operative it may make life easier for you for a couple of nights.
IMHO you'll find DC to be a fantastic day for her, Dolphins and special needs kids seems to be a magic formula. Obviously it very much depends on the individual child, but the experience seems to be something that works very well for many of those children.
