Deepfakes of an actual child should be considered defamatory use of a person’s image; but they aren’t evidence of actual abuse the way real CSAM is.
Remember, the original point of the term “child sexual abuse material” was to distinguish images/video made through the actual abuse of a child, from depictions not involving actual abuse – such as erotic Harry Potter fanfiction, anime characters, drawings from imagination, and the like.
Purely fictional depictions, not involving any actual child being abused, are not evidence of a crime. Even deepfake images depicting a real person, but without their actual involvement, are a different sort of problem from actual child abuse. (And should be considered defamatory, same as deepfakes of an adult.)
But if a picture does not depict a crime of abuse, and does not depict a real person, it is basically an illustration, same as if it was drawn with a pencil.