That's quite a difficult question. I'm assuming that you're talking about pornography where it is explicitly stated that those involved are all over 18 (and they are), but the images or whatever imitate child porn. Obviously, if there are claims that the actors are under 18, it would probably be illegal as well.
On the one hand, as has been pointed out, no children are actually harmed in the process of making the porn. And perhaps, as disgusting as it is, it could reduce the demand for actual child pornography.
On the other hand, it is encouraging something that is appalling harmful to innocent children, and is a huge problem in our world. I'm not saying that it could make someone become sexually attracted to children, but that it could increase the desire of paedophiles to view actual child pornography or to start or continue to abuse children themselves.
Because it is pornography specifically, it's different from other media in that films involving murder, rape or whatever are not made solely for sexual enjoyment. Pornography is, so simulated child porn is actually encouraging child abuse though films etc. can include something without encouraging it in sucha way.
Other types of porn (with the exception of that involving animals if it exists

), no matter how repulsive some people may find them, even if animated, could all be performed in reality with consent. Simulated child porn could not, because children cannot consent to sexual activity.
So I guess I'd have to say it should probably be illegal.