I would think that at that point, it would fall under federal obscenity laws.
Specifically from that page: "visual depictions, spoken words, or written text". If a minor was at a private event and exposed to either materials or live acts of a sexual nature, that seems like it would be a "visual depiction". Officially, the courts apply the Miller Test of Obscenity, but I would think a live sexual act would count just as much as a video.
Per the same page: "Federal law strictly prohibits the distribution of obscene matter to minors. Any transfer or attempt to transfer such material to a minor under the age of 16, including over the Internet, is punishable under federal law."
There may be specific case law that defines whether a live act qualifies as "matter" and/or whether viewing constitutes "transfer", but at the surface level, I don't see why they wouldn't.