"Max Hardcore" did time in prison for this very thing. Though his videos use all over 18 actors, in one of them the "actress" claimed she was 12 (was in fact in her 20s.) So that statement alone classified the whole video as child pornography.
That's obviously not something which should be illegal since the absurdity stands apparent if you imagine the cast of "90210" (who were in their 20s and 30s) claiming to be minors. Though I didn't follow the series, I would assume there were scenes of making out or even sex which this kind of law would make child pornography even though the sex is simulated, and the actors NOT minors.