There is a really fine line between childhood, adulthood, science, laws, and cultures. It depends on who you ask as to what defines a child as being an adult.
Child pornography is defined in the US, as what its 'defined' by, we can't just simpley change that. Unfortunately "law" tends to over rule science, and cultural aspects. Law makes its best attempts at protecting childern, which is the purpose it serves.
Art has a long history of having naked people it, some people draw things peacefully naked, others draw them in a vulgar way. I feel in some cases (modern cases mostly) when people depict childern in sexual acts in art, they aren't doing it for the 'concept' of art, they're doing it for (self) sexual pleasure. Art is supposed to be art, its supposed to be beautiful, something to enjoy, I feel people may abuse it. Very unfortunate.