Ref T12074. Allows querying for project by direct parent (find only immediate children) or any ancestor (find all descendants) using the API.
There's no proper web UI for this since I'm not sure how useful it is, but you can /project/?parent=PHID-PROJ-... or /project/?ancestor=... for now. We can add UI later if/when use cases arise, but it's not immediately clear to me that this is useful to do from the web.