In the canon, I think they just end up looking like the grown up version of themselves. My reasons are because in the movie, Mary (the oldest) is played by the same actress when she transforms and in the comics (Kingdom Come, I think) years in the future one of the Justice League members sees a person who he thinks is Captain Marvel, but it’s just grown up Billy Batson.
Better yet, Luthor has Batson in his entourage, so everyone thinks Captain Marvel is rolling at his side all the time. Until Batman realizes it’s just… Billy Batson.
In the canon, I think they just end up looking like the grown up version of themselves. My reasons are because in the movie, Mary (the oldest) is played by the same actress when she transforms and in the comics (Kingdom Come, I think) years in the future one of the Justice League members sees a person who he thinks is Captain Marvel, but it’s just grown up Billy Batson.
Better yet, Luthor has Batson in his entourage, so everyone thinks Captain Marvel is rolling at his side all the time. Until Batman realizes it’s just… Billy Batson.