If something is reported to 5P, they decide whether or not it should remain from the site or be purged, based on whether or not they personally believe its sexuality outweighs its artistic merit. Additionally, anything involving children in sexual situations is blanket banned regardless of artistic merit. At least, that's how they describe it in the rules. There's a lot of weird exceptions, which is natural given the nature by which the decisions are made.
Out of morbid curiosity, is there any scenario you can name involving children in sexual situations that actually have artistic merit?