As far as I know, the research seems to indicate that indulging pedophilic fantasies actually increases the chance that a pedophile will harm a child. In theory I don't think it's wrong for someone to fantasize about children in their own mind or write stories or draw erotic pictures involving children, but in practice, I think it's very unwise for a pedophile to do so. I know when I watch porn or fantasize about men, it makes me crave sex more. On the other hand, if I don't have sex for a long time and don't watch porn or indulge in fantasies, my craving for sex diminishes. I imagine it's probably the same for pedophiles.

Also, again as far as I know, pedophiles are not simply like regular people but attracted to children. There seems to be some differences in their brains. They have an extremely high recidivism rate, and I've read that this is because they have problems with impulse control in general.

So, let me ammend my statement:
a)someone who is attracted to children in real life, has fantasized about a real child that they have seen, or has ever touched a child inappropriately or come close to touching a child inappropriately, should most likely try not to fantasize about children or write erotic stories or draw erotic pictures of them

b)someone who has never touched a child, is not attracted to children in real life, and has never felt the urge to touch a child or watch child porn, is probably safe writing erotic stories and drawing erotic pictures of children. This is typically done in the context of various fandoms. But even in fandoms, it is rare to see true pedophilic erotic (involving pre-pubescent children under 12 or 10 years old). Most underage stories in fandom involve young teens -- but teens nonetheless. And that's not even pedophelia.

So ethically it is not wrong to fantasize about children unless you suspect that it will whet your appetite for harming one in real life. And for a lot of pedophiles, that's simply how it is.