As far as I know, Plex doesn’t host their own content. The Plex Movies & TV are hosted by the providers (Warner Brothers, Lionsgate, etc), and it’s all mainstream hollywood stuff, none of them make porn, so, I don’t think it’s that. What BigWheel said seems accurate, but that begs the question, if it’s prefetch metadata, how on earth is it grabbing what appears to be an entire movie? I tried to play one of the toy story movies in that section and it clearly gives a length, and that length appears to be correct when comparing to my actual library, but it doesn’t actually play, and I know for sure Plex hasn’t gotten a contract with Disney (Disney is pulling out of all of their streaming contracts now that they have Disney+ up and running and they own Majority share of HULU).

Edit: The good news is, it only appears to be a problem on app.plex.tv, I tried the Plex app in Windows, the Plex Web app from localhost, and my FireStick, and it doesn’t show on any of those, so… Hopefully your kids aren’t watching plex from a web browser using app.plex.tv