I'm afraid I don't know any makes, as I don't have any kids, but it might be good for you to go to a local store that sells them, and test them out with your DD and DS. You can then order the best one online, if that's cheaper. The main things to consider when buying a stroller for WDW are:

Will the kids fit comfortably (without getting in each other's way / kicking each other / squabbling etc?

Will it fold up well to fit on the buses?

Is it too wide to fit through some of the ride queues?

Does it provide enough shade for them?

Will the fabric 'breathe'? If not, they may get heat rashes
There are probably more things, but none that spring to mind immediately.
Another thought occurred to me, might you be able to get a pediatric wheelchair for your daughter through your insurance? If you can, she would then have that to use whenever she needs to, and might be able to push herself a little (e.g. in shops), if her condition permitted. She might also be happier using a wheelchair than a stroller (I don't know whether this bothers her or not). You could then get a lighter-weight, less bulky, single stroller for your son. Any time one or the other is not being used, it can be parked in the stroller parking. Of course, this would only work if you have two adults who are willing and able to push. Might this work for you guys?