I think you're going about this wrong. The difference is what harm is "valid"; if I laugh at you and humiliate you, it may indeed make you upset and traumatized, and I think it may be morally or ethically wrong for me to do that - but should that mean that there is a law against that?

At some point we draw the line, and in almost all other respects the line is drawn elsewhere - in that mere knowledge of an act occuring is not sufficient to make that act illegal.

The equivalent here would be the fact that the robber is in the gas station, I think? Should the robber not be allowed to be anywhere where he may even be seen by you? In the same way, even though the abuse of the child was problematic, should someone who views the abuse be disallowed from doing so? And further, there is even more disconnect here, making your analogy much weaker - what does the victim know of the instance in which someone views that recording?