The ADA (Americans with Disabilities Act) has
certain requirements (by law, for the entire country) for the
size and number of handicapped accessible toilets in public restrooms, not who can use them.
The basic requirement is that 5% of the stalls (but not less than one stall) must be handicapped accessible. This means that in a large public restroom with 50 stalls, only 3 are required to be HA. A small restroom with only 3 stalls total, would be required to have 1 HA stall. It doesn't make sense to "reserve" one stall in those little restrooms to be only used by people with disabilities (doing that would mean long waits for everyone). But, in larger restrooms when other stalls are empty, it does make sense to choose another empty one if you don't need the features of the HA stall.
Some of those HA stalls are required to be accessible to people who need grab bars they can use to walk on both sides of the stall (meaning the stall is 36 inches wide). This means it is too small for people with wheelchairs unless they are independent.
The minimum requirement is for the HA stall to be 56 inches long; many wheelchairs are up to 48 inches long from the back to the front of the footrests. That means many of the HA stalls are not accessible to people who need their wheelchair in the stall with them (or older children/adults who need an adult in the stall with them).
If you think it is hard getting into a regular stall with a child, think of getting into a 36 inch wide by 56 inch long stall with your child whose wheelchair is 24 inches wide and 48 inches long - the footrests touch the front of the toilet and it practically takes an engineering degree to figure out how to arrange everything so the child can actually use the bathroom. And then, add that the child can't stand without assistance and you can see the difficulty in using those stalls.
Most of the HA stalls at MK and Epcot only meet the minimum requirement, which means that they are not accessible for most people with disabilities who need their wheelchair or an assistant in the stall with them.
Most of the restrooms in AK and MGM have at least some larger HA stalls (many much larger stalls with sinks in the stall), so the "wheelchairs don't fit in the stall" problem is not as bad in those parks.
Actually, people call those restrooms Family Restrooms, but they are
Companion Restrooms at WDW and are are labeled "Companion Restrooms". They are WDW's solution to the "HA stalls are too small" problem. The only places that their location is listed are in the Guidebooks for Guests with Disabilities. There are not enough of them to function as Family Restrooms, so they are only "publised" in the disability information.
At MK, for example, there are 6 or 8 of them. (including one in First Aid which is a bit small to comfortably bring a wheelchair in. Also, I prefer not to use that, as it is the only restroom available to someone who is ill in First Aid.)
Although the handicapped stalls in the restrooms may look large, almost all of them at MK and Epcot are too small to bring a wheelchair into the stall and close the door or for an adult to assist another adult or large child.
Some older people (like adults or older children who are disabled) need someone of the opposite sex in the restroom with them (and because of that, can't use the men's or ladies' rooms). The only places available that some people can use are the Companion Restrooms.
For people like my DD, since MK has only 6-8 Companion Restrooms, that means there are only 6-8 toilets in the whole park that she can use. Not 6 - 8 restrooms with multiple toilets in each one; 8 or less toilets. Total. In all of MK.
To some people, it may be just another restroom out of many (or another stall out of many in a restroom) that they can choose to use. To many of us, we have no choice. That restroom (or that handicapped stall in the restroom) is the only one we
can use.
I usually say that anyone who is going to use the HA stalls or Companion Restrooms should keep in mind that for some people using them is a choice, but for other people they are a necessity. I personally would appreciate it if the people who have other possible choices would consider their choices before deciding to use them and realise that some don't have any choices.