While your rationale is sound, it is not the only one used when various governments form their child pornography laws.
In Canada not only are depictions of sex with 'simulated' children, which includes anime, considered child porn, but even text - stories of sex with or between children is also.
And I repeat, that is a weak basis for a law. We had a fellow here who was on parole for a child-molestation charge. As a condition of parole, he was not to possess child-porn (duh!)but he was found with a diary filled with his own written musings on sex with children. He was arrested for a parole violation and it was held up at trial. On appeal he was freed because, essentially, a person's diary is simply a repository (in this case a septic tank) of one's own thoughts and one is free to think what one will.
Key to this decision was the absence of any real children in any aspect of the offending material.