Hi Gerik,
There isn’t a display option for sub-categories or child_of. It is something we’re looking into.
It’s possible to use the generate_element_display filter to set custom display conditions.
Reference: https://docs.generatepress.com/article/generate_element_display/
However, it would mean you have to “hard code” the parent category terms in the filter, so if you added new parents then you have to edit the code.
Let me know if that’s an option and we can see if we can provide a snippet for that.