ipvfoo will show you the hosts contacted by Plex Web, some of which seem to be tied to the streaming services - vod.provider.plex.tv, webshows.provider.plex.tv, etc.
You can try blocking the applicable hosts for your kids’ devices during the desired time period.
I’d like to block the online media sources only during night time but let it work during the day so this would not be suitable as every night I would vave to manually shut it off for each kid or the local media would also cease to play.
Bacically I’d like to have a seperate time period for online media and my media of which plex is not capable of.