Seen recently in a highway rest stop bathroom:
(Yes, this is the first time I've seen a need to take a picture in a rest stop bathroom, for anyone who may be wondering.)
The sign on the stall door said "Mommy & Me." It was about the size of a standard handicapped stall, but as you can see, it has not only a regular toilet but also a small one for a young child and even a seat (the gray thing on the wall) in which you can safely restrain an even younger child.
Where was this when I was traveling with three kids under the age of five? Fabulous idea!