In the US in particular, it doesn't matter if the underaged subject exists or not, but I think you'll find the law much more loosely enforced in cases where the subject of the pictures doesn't exist.
> it doesn't matter if the underaged subject exists or not
Are you sure this is true? As weird as it is it seems pretty ridiculous to suggest that someone drawing a sketch could be guilty of the same thing as someone abusing a real child.
They're both illegal under the Child Pornography Prevention Act of 1996:
> The Child Pornography Prevention Act added two categories of speech to the definition of child pornography. The first prohibited "any visual depiction, including any photograph, film, video, picture, or computer or computer-generated image or picture" that "is, or appears to be, of a minor engaging in sexually explicit conduct." In Ashcroft case, the Court observed that this provision "captures a range of depictions, sometimes called 'virtual child pornography,' which include computer-generated images, as well as images produced by more traditional means."
> The second prohibited "any sexually explicit image that was advertised, promoted, presented, described, or distributed in such a manner that conveys the impression it depicts a minor engaging in sexually explicit conduct."
"Computer-generated" images was a remarkably prescient thing to include in 1996, since it criminalizes deepfakes and GAN images.
I think the standard argument is that even though actual minors aren't harmed by the creation of such images, the images trigger pedophiles to become more depraved or something. There's probably an element of the old obscenity laws to it - real or not, child porn is obscene, and obscenity isn't covered by the first amendment without some artistic merit.
I'm not sure of case law around enforcement of this.
Are you sure this is true? As weird as it is it seems pretty ridiculous to suggest that someone drawing a sketch could be guilty of the same thing as someone abusing a real child.