。
She gave the crying baby some milk.
Literal
She [topic-は] crying baby [to-に] milk [object-を] caused-to-drink.
飲ませる is the causative form of 飲む — literally 'cause/let to drink.' In contexts of feeding a baby, it's the standard verb: you don't drink for the baby, you 'have' the baby drink. ミルク here, written in katakana, almost certainly refers to baby formula (粉ミルク) rather than ordinary cow's milk; that's a meaningful split in Japanese — 牛乳 is the everyday drink, ミルク tends to mark the baby/coffee/dessert sense. The に on 赤ん坊 marks the causee — the one being made to drink — which is the standard pattern for the causative when an object (the milk) is also present in the clause.