To me Child Porn is that of a child being photographed in a sexual manner with the
intent of conveying sexuality. A baby or even a 3 yr. old sitting naked on the living room rug just sitting there looking happy or crying or whatever but nothing that really could be construed as sexual (except by the worse of deviants) isn't porn.
I've seen photos of a nude woman holding her nude daughter of 8 close to her body in an embrace and called it art, nothing explicit was shown even the girl's breast/nipples were covered by the mother's arm and it was all above the waist. But nothing could've been interpreted as sexual... at least as I saw it. But others will.
It is a fine line but it all boils down to, I believe, with the intent of the photograph. Was the photographer intending to elicit arousal upon whomever might view it, was the photographer intentionally trying to convey sexual overtones. That is porn.
The pictures so far, to me show only naked children, the backgrounds and props are part of the photo and thus it's art for art's sake and intended to convey whatever message the photographer wanted to say.
In the case of Pretty Baby (and soon to be released film starring Dakota Fanning in a supposedly explicit rape scene
http://en.wikipedia.org/wiki/Hounddog_(film) ) Shields was wearing a body suit and the male actor (Keith Carradine) was himself wearing a flesh colored brief which covered his genitals. Again the nudity and sexuality of that cannot IMO be interpreted as Child Porn because of the intent of the story line was not to arouse it's audience but to tell a story about child prostitution in the early 1900's. Granted, the writer(s) and director
could've have written/directed/shot certain scenes to imply nudity here, sexual contact there and so on but the film might have not garnered the attention and boosted Shields to stardom. But they still couldn't been interpreted as porn.
One man art is another's porn as the saying goes.
However; when it involves children then a line must be carefully drawn. What are you trying to say with that photo, what are you trying to get your audience/viewer to feel? That makes a difference, a BIG difference.