At WDW, they are called "Companion Restrooms" and they are not listed on any maps (probably because there are not enough of them to be used by large numbers of people as Family Restrooms). Another option would be to use the bathrooms in the Child Care Centers (there is one in each park).
At MK, the wheelchair accessible stalls inside the bathrooms are not large enough to have a wheelchair in the stall with you and close the door. I know they look large, but once a wheelchair is in, either the door doesn't close at all or, with the door shut, the footrests are resting against the front of the toilet. If someone needs their wheelchair or an assistant in the stall with them, it's not possible in these stalls.
WDW's way of dealing with this was to make some Companion Restrooms. At MK, there are 8 of them and they are the only stalls in the whole park that someone who needs their wheelchair in the stall with them is able to use. I am not saying that only people with disabilities can use them, just that if you have other choices available, please remember that these are the only 8 stalls in the whole park that some people are able to use and consider your other choices.
Epcot is a little better because it was built more recently and had more room for adding facilities. At AK and the Stdio, most of the restrooms have large anough stalls to accomidate a wheelchair.