One problem is that all these distinctions are highly dependent on context. Let me give you examples.
If a parent takes one photograph of a little boy or girl doing silly things half-naked, this is "cute". If, however, a whole series of such photographs is sold online, then it's child porn.
If one sees "Venus chastising Cupid" in a museum, this is classical art and they take groups of children to see it. If one sees a manga drawing of a naked, buxom woman, spanking a naked child with the buttocks turned to the eye of the viewer (which is an exact factual description of the above classical art), then this is child pornography.
If some random Internet user writes stories involving minors getting raped, mutilated and other gorish stuff, that person risks prosecutions in many countries including the US. If Alain Robbe-Grillet, member of the French Academy, respected writer, does so, then the book is sold in all good libraries (possibly with a word of warning to the reader).