"ʻO wai kāu hiapo?"
Translation:Who is your first-born?
I entered "Who is your first born child?" and was marked incorrect, probably because hiapo assumes the first born is a child. But I think "first born child" is a common translation.
Mahalo for posting this to the forums! Both "first born child" and "first born" should be accepted once the Duolingo system updates.