Question, what is it then?
The sexual gratification has to come from something yes? Something intrinsic to the child character. It can’t just be the flat chests, because you can get that with adults, nor the height and size because again small adults exist. Even the “youth” aspect can be accounted for with young looking adult characters, or at least teens.
Therefore, it is logical to conclude that the sexual gratification comes from the character intrinsically being a child. Otherwise, they would not be getting their gratification from it yes?