The goal is to create a user in Jellyfin that can only see media files that have a given tag, say “kids”, and that tag was set inside Radarr, Sonarr or another *arr application.
*arr Configuration
Within Settings > Metadata > Kodi, check “Enable” and “Movie Metadata”.
Then, when adding or editing a movie, add a tag to mark media that children can watch, like “enfants”.
This will create a .nfo file that includes the tag:
movie>
<
...tag>enfants</tag>
<
...movie> </
If you add this tag to an existing movie that is already synced with Jellyfin, click the “Refresh & Scan” button.
Jellyfin Configuration
In Jellyfin, under Administration > Dashboard > Users, create a user and in “Parental Control” tab, only allow items with the tag chosen above:

The tag seems to be case-insensitive.
That’s it!