It's a beautiful and artistic photo and I see nothing lewd about it. But. My problem is how openly candid the photographer is of *who* the subjects are, given the photo's intense intimate and emotional strength. Photographers can and do shots like these, but it's not so common that they will publicly identify the subjects, out of respect for their privacy. The message is the mood and feel, not the "who".
Heather Whitten is a photographer who's body of work on occasion shows herself and her children in partial or obscured nude; this is not a one-off exception. Again, this not about child nudity in art, but that she openly identifies her subjects, her children, by name and city on public web forums with these pictures linked.
I see this as a mentality common of the Facebook generation, where people are finding it okay to live under a social media microscope, and pulling what would traditionally be small audience exhibit works into their spotlight. The nature of her photography, I wouldn't even bat an eye if I went to a gallery and saw her work. It's art. But her gallery of choice is the wide open internet, and once that art is on display, the exhibit is forever. Her children will not have the choice and ability to avoid these images as they get older.
Using that photo in her photography class. I have no problem whatsoever with that.
Using that photo on a photographer's blog about how people misunderstood it. Mmmm, that's your choice, but....
Allowing the AP to identify your son and circulate the photo and story to dozens of highly-trafficked international news sites. I see that as a problem.