>but it is likely that it does so indirectly by creating demand for actual CP
Until there is evidence of that, it seems strange to say that's a conclusion at all, or that it's likely. In fact, Patrick Galbraith's work on how users of fictional CP actually interact with their materials suggests exactly the opposite: they have developed a form of sexuality for the representations themselves - the more fictional, the more real to them. To add to that, the research I've read shows that pornography is not associated with increased rates of sexual assault or rape. If the link isn't even there for regular porn, what would make you say it's "likely" there for cartoon representations?
Furthermore, do we apply this standard elsewhere in society? If there were no research on the link between video games and violence, would you conclude that it is "likely" it indirectly creates a demand for videos of real violence? Is the fact that someone might be spurred on to do a different act sufficient to illegalize the original act which spurred them? I would say not, unless we were to illegalize drugs (for example) because those who partake are likely to be violent? Should we prosecute cannabis users because they create demand for drug cartel products which finance murder? Apple users because they create demand for the appalling treatment of Chinese workers?
Most researchers on the philosophy of law in this area (e.g Suzanne Ost) while obviously opposed to actual child pornography find no philosophical basis in the harm principle to illegalize fictional representations. As such, it's not reasonable at all. You need to justify why you think it's "likely" and just what that standard means. If it cannot be justified, then we may say that any likelihood is sufficient to illegalize any material, but I doubt that's a conclusion you'd want to follow to its extreme.
Finally there's the issue of penalties - isn't punishing someone for something they might do (access real CP) unjust? This seems shockingly close to the notion of pre-crime. To avoid that, you'd want to look more at punishing the creators of the material rather than those who have consumed it and harmed nobody (other than themselves perhaps).
If there is no cultural understanding of the real people themselves, how they read the text and how they interpret it, any law is based merely on speculation and I must stand firmly against such speculative laws.