I actually didn't downvote you, but the problem is that you're making a lot of assumptions when you say "child porn", and you're ignoring a key way that that child porn is already treated specially by the law[0].
If you're 17 and your 17-year-old boyfriend/girlfriend texts you a nude selfie, that's child porn. If your phone is confiscated before you even open the text, you're guilty of possession of child porn.
If you're 5, and your parent or relative sexually abuses you and records the events on film, that's child porn even if they don't distribute it. If they do, everyone who receives it is in possession of evidence of your abuse.
Child porn is evidence of a crime, but the question is, crime by whom? In the first case case, it's not clear morally that the person who unknowingly received a nude selfie from an underage person committed a crime, even though legally the law treats both cases as identical.
[0] For contrast, if you are found in possession of drugs, you can theoretically make the defense that you didn't know you had them. It's a weak defense, but it is legally possible - with child porn, even if you succeed in convincing the judge/jury of that fact, it doesn't affect your case one bit.