I found this interesting, looking at the Firebase API. I guess one could get the min/max item ids for the child posts, and query the range of ids (startAt(), endAt()).

Van Puffelen answers some of this here, but he doesn't get into specific queries: https://www.youtube.com/watch?v=66lDSYtyils

He also mentions here that Firestore offers a bit more for WHERE type clauses over Firebase:

https://stackoverflow.com/questions/26700924/query-based-on-...