My older DD uses either her power wheelchair or her manual wheelchair independently at home. So far, we haven't brought the power wheelchair along to WDW (various reasons, including air travel). She is not capable of pushing her manual wheelchair for much distance, so doesn't use it outside the house. If your son is normally able to manage for school and shopping trips, etc, he should be able to do a lot of his own pushing.
For the 4 year old, a lot depends on her and how you plan on touring. If you are going slow and not planning back to back long days, she may not need a stroller. When we went in March, we had my niece and her 3 year old. The first 4 park days, we didn't have a stroller and the little girl did very well. The end of the 4th day, she said her legs were "all out of walk". My DH took them to Target and bought a stroller, which was used after that. But we didn't really take a day off, so she had back to back long days. On several days, we had DD in her wheelchair, niece's DD in the stroller and my FIL using an
ECV. We did feel a bit like a train, but you do what you have to do.
You will see lots of people with 2 or more strollers, so you won't be the only family pushing more than one thing.
