Well, what makes a crime is a law stating that such-and-so action is a crime, so I suppose I'd say that, since a
legislative body classifies an action as a crime, it is pretty much by definition exactly that. We're discussing whether this specific action
should be a crime, which is a separate conversation altogether, and one I think best carried out without reference to some sort of nebulous Platonic ideal of crime, to which you seem implicitly to appeal.
You're also underplaying your hand here somewhat, I think - either that, or you find your own arguments unconvincing. You've already conceded, in your concept of "indirect harm" (still harm, but less so?) and the weaksauce relative-injury argument which constitutes your last paragraph, that the distribution of child pornography does inflict ongoing harm on those abused in order to produce it - but you don't think that that harm suffices to justify the injury inflicted upon those who engage in such distribution, but not such production, in the course of being convicted of and punished for that crime. That the former are innocent, and the latter are not, seems not to move you; your concern appears instead to be purely utilitarian.
I don't suppose it would be surprising if you were indeed unconvinced of this - it strikes me as a very difficult position to defend! But I'd be interested to see whether and how you do so, nonetheless. Or perhaps I've misgathered your point?