Just so you know, the better way to do this is to limit the kids by rating than by a separate library.
However, if you’re simply not interested in seeing the kids/family movies in your main move library then this makes sense.
So, the only thing you need to make this happen typically is the correct filenaming system.
/kids movies folder/Movie Title (year)/Movie Title (year).ext
For instance:
D:/Kids Movies/Aladdin (1992)/Aladdin (1992).mkv
This is the way all your “Movie” Libraries should be structured for Plex Agents to pick everything up correctly. You can use a tool like filebot to create your structure for you en mass. Once this is accomplished, you may have to do the Plex Dance