Short answer
The sponsor must usually be at least 18 and be the eligible parent, or that parent's cohabiting spouse or de facto partner.
The sponsor must be an Australian citizen, Australian permanent visa holder or eligible New Zealand citizen.
The sponsor is usually the parent referred to in the Child visa criteria, or that parent's cohabiting spouse or de facto partner.
The sponsor undertakes to assist the child as needed with finances and accommodation for two years after the child's first entry to Australia.