If paternal is "relating to someone's parents", and fraternal "relating to someone's brothers", is there, or why isn't there, a word for "relating to someone's sons", i.e: sunternal Sentence example: "sunternal love"
There is filial, from Latin filius "son" and filia "daughter". So filial love should work in most contexts.
Perhaps "progenic" would also be a suitable candidate. Progenic is the adjective form of progeny - which means "child or offspring".