expression, godan verb (-mu)
to be breastfed; to drink breast milk
Used for babies or young children breastfeeding. The subject is the child, not the mother.
赤ちゃんがおっぱいを飲んでいる。
The baby is breastfeeding.
うちの子はよくおっぱいを飲む。
My child breastfeeds well.
授乳する is a formal term for 'to breastfeed' from the mother's perspective, while おっぱいを飲む is a casual, child-centered expression.
Phrase composed of おっぱい (child's word for breast milk) + を + 飲む (to drink). The exact origin of おっぱい is uncertain, but it is a common baby-talk term.