While it may be unlikely that anyone would be convicted of producing kiddie porn because of innocent naked pictures, being accused is often enough.
This couple's children were taken away.
I've heard about other cases, too, but I don't have links. There was one woman who actually went to trial for innocent, IIRC, SEMI-nude pictures of her girls. Yes, I'm pretty sure it was pictures of a topless five year old or something like that, but I might be wrong, because I can't find a reference.
I think this is a TV movie about it.
So it happens. It's really very subjective, and when we rely on people's judgement to determine whether something is a crime or not, someone somewhere is going to be screwed at some point. I'm not saying it's avoidable or that there are objective means to determine what's porn and what's just nudity. It's just that that whole grey area can really be a minefield.