Historical context and perspectives regarding nudity and it's purposes are what matters in situations like this. It's the same sort of argument we see regarding old photographs of young children taking baths together; most of the time it isn't done with the intention of creating pedophillic content but, instead, parents highlighting the innocence of childhood and family hallmarks in a way that seems creepy and weird after the fact. Of course, that kind of shit shouldn't, in my opinion, be shared online for the world to see for many, many reasons; not in the least of which that material being collected and shared among pedophiles and strangers for their own personal reasons. But most people aren't going to balk if they find photos of themselves at age five playing naked in a sprinkler if it's in a family photo album.
Art is difficult to gauge because at the end of the day a big question is whether or not a child was directly exploited or not to create such content and if the intention was to sexualize their presentation. Given most of Michelangelo's children (and women) were proportioned more like babyfaced adult men, I think it's safe to assume that his reference material wasn't all that accurate if we're talking about the specific topic at hand; there were laws regarding nudity of women at the time being seen as problematic within the context of the church, but nudity involving children was not viewed the same way to my knowledge. And yet, Michelangelo's cherubs are suspiciously six-packed and muscular.