Incidentally, it is common on "adult" sites, that operate on a similar basis to Renderosity, to forbid ALL depiction of children, clothed or otherwise. I think this is perfectly right. The argument is, that on a site the very purpose of which is overtly erotic, any image appearing is eroticised by implication.

The other side of the coin is that for a site that is not erotic (like Renderosity) the depiction of nakedness is simply the representation of the natural state of mankind, whether of children or adults. In image of a child with no clothes on should cause no more trouble than the countless paintings of the child Jesus that you can see in any art gallery.