— and the latter reminds me that the parent | child | grand-child arrangement for these OSAF forums changed significantly around the time that <http://n2.nabble.com/IETF-f3136371.html> and its children were created/arranged.
This is a misunderstanding. Even if you see [Everyone, Everyone, Everyone] selected, the bottom block says "Use my settings, AND parent settings". So if the parent forum is private, then child forum will also be private. You break the restriction if you select "Use my settings, OR parent settings" or "Use my settings only". So you don't have to redefine everything again. Just make sure you have the "AND" option selected.