I think, there is no general answer to this.
"Naked" can mean a lot of things - just the human body without clothes, as it's depicted since the old greek (although they also had some really sleazy stuff), can't be a bad thing. Also for children - there's no "danger" in it.
Sexual depictions are another case. It's harder to hold an aestethic standard, because they very easy can look like cheap masturbation material. So I understand, that they (who may indeed in some cases confuse the sexual developement of children - just, because they are a step, that needs other steps before, they don't have taken yet) are not allowed everythere.
To draw a clear and absolute line between the two kinds will mostly be impossible.