All pornographic depictions of minors in digital art, cartoons, AI or otherwise, are banned if they are:

Obscene, OR

Appear indistinguishable from real minors.

The key legal test is whether the work is obscene (per the Miller test: community standards, offensiveness, lack of serious value).

Purely fictional, non-obscene art may still be protected, but most sexually explicit depictions of minors will be considered obscene.