I think we need to recognize that even if pictures of naked children doesn't turn us on, it does turn on someone else. It doesn't matter how innocent of harm the intent of the painter or photographer was. The reality is that the end user twists those representations for their own use and urges. Heck, I remember as a kid talking with boys who beat off to the pictures of nude people in National Geographic.
A pedophile is attracted to children under the age of consent and the number of them is put at around 25% of the population (3% female, the rest male). Based on the large numbers of child sexual abuse, pedophiles aren't happy to just stick with those picture. They act and they act in ways that are harmful to children.
Consent is a bedrock concept for our community and it really disturbs me when those that can't consent are exploited. No person at the age of 12 can be considered "well-adjusted and mature." Certainly the parents should protect their child better but society at large has an obligation to step in when that protection is lacking.
I don't think the pictures are art.