The laws concerning anything that could conceivably be called 'child pornography' tends to be a minefield, but even with today's ruling, the Supreme Court's general stance is that if the people are not real, or those involved are not actually minors, then it is legal, regardless of the content.

So, as far as CP laws are concerned, with regards to any upheld laws that I know of, the following are LEGAL:
-Drawn representations of minors.
-Written stories about minors.
-Adult actors pretending to be minors.
-Films involving minors in


However, the following are ILLEGAL, or in such a gray area that prosecutors will go ahead and raid your house even if it isn't.
-Minors in simulated sex acts.
-Photoshop'ed or digitally altered pictures of minors to show them in sexual situations.
-Showing any interest in obtaining CP, whatsoever

You should be careful, though-- even if you aren't violating any CP laws, prosecutors have complete discretion to prosecute you under catch-all obscenity laws, even if nothing you've written is explicitly illegal. In general, I'm recommend staying away from using characters under 18, and scenes of heavy and fatal violence. Those tend to be what attract the most obscenity prosecutions.

Be careful when traveling. Canada and UK are a lot less nice considering fictional portrayals of... everything... so laptops can get searched and seized at customs.