Several things when dealing with fictional (AKA drawn/cartoon) artwork of 'child porn'.
A: Even now, what actually defines 'Child Porn' (And obscenity in general) is fairly loose. We've had teenagers send naked pics of themselves to their boyfrienRAB, and get charged with sending childporn. Recently, a couple was investigated because they send photos of their children bathing to Walmart to get processed. I have a big problem with any law that essentially uses a definition of 'I'll know it when I see it' as what's porn will vary from person to person.
B: It harms no ones. Just because you dont like that sort of stuff, and find those that do creepy, it's not a reason to ban it. There's often a common argument linked to this that goes 'Well, if they enjoy fictional portrayals of this, they might want to try it for real!' If you ARE considering that train of thought, you might want to consider why you think this should be banned and other fictional/fantasy works such as torture porn or just fantasised crime sprees (Hello GTA) shouldnt.
C: Sort of linked to A...but there's a strong undercurrent of sci fi and fantasy in a lot of anime (and thus hentai based on it). Take Negima for instance. Chachamaru is...what 3? Yes she has the body and mind of someone much older. Eva, on the other hand, looks to be 10 or so, and yet she's something on the order of 1000+ years old. If someone made porn of one of those characters, which would be child porn? Do we look at their chronological ages, or do we rely on 'apparent' age?