What do you mean by default Movies and TV Folders? We don’t have a “default”.
You can create separate libraries for your kids stuff and then create a user account for your kids with access only to those libraries.
If you are referring to the WD My Cloud devices, then that’s not possible. You would need a Plex Pass and then you could use a sharing label or rating restriction.
Early 2021 clean-up: implemented (servers can have as many libraries as a user needs; access control via parental controls, restriction of shares within a Plex Home/managed users)