Interesting conversation and a healthy one to be having especially in this forum. As photographers, professional or amateur, it is really vital that we understand the laws
before capturing images of the human body.
I do not know Hensen's work so I can not comment directly to the exhibit in question, however I feel I can shed some light on the topic in general at least from the perspective of US law.
First, by the law, pornography is more than merely depicting a sexual act. If the
intent of the depiction is appeal to prurient interests - to arouse sensual or lustful emotions - then it is pornography.
Note the word "intent" as it is key form a legal standpoint and is also the reason there is so much debate on the topic. Showing a homosexual man an image of a nude woman engaged in self pleasure may not illicit prurient thoughts from him, however this does not mean the image is not pornographic. It is pornographic because the
intent of the artist was to illicit such response. So just because an image does not arouse YOUR prurient side that doesn't mean anything. This is why intent is difficult to prove legally even though generally we can discern it easily through simple observation.
Secondly, we have the term "obscenity". Legally this is all over the map in the US as it is so very much a subjective definition. However, in the US we have the "Miller Test" (google it) which sets forth a 3-prong test to determine whether a depiction is considered obscene or not - based on "community standards". So depending on the community you are located in there can be a wide variation of what is and is not obscene.
Now then, when it comes to children US law is pretty straight forward in so much as the above two definitions allow. A "child" is defined as anyone under the age of majority and in the USA that age is eighteen (18). US code 18 U.S.C. 2257 requires that the author/producer retain legal identification proving that for all persons depicted in any "pornographic" photographic or video production are/were above the age of majority when the images were captured.
Therefore, depictions of children in a pornographic setting are illegal, period. They are also considered obscene by most community standards under the Miller Test.
So even though things look fairly concise legally, because key elements of measuring works are subjective it is actually a pretty big mess. So in light of this my non-legal advice is: If it looks like a duck, walks like a duck and sounds like a duck then you are wise to assume that it is in fact a duck.
Henson's exhibit may very well be well within common acceptable norms of decency and he is merely being unjustly scrutinized. Then again, artists do love to push the envelope and while I'm all for this I also subscribe to the truism of "discretion is the better part of valor" and when it comes to blurring the lines between artistic nudes and pornography with children in today's world ... well that's one battle I'd just as soon sit out.
edit: btw, I am not a lawyer nor do I play one on TV. I merely have some exposure to the issue from consulting work building web applications. Do not take anything above as being the gospel truth - that's what 1st Amendment attorneys are for.