。
She fed the baby milk.
Literal
She [topic-は] baby [to-に] milk [object-を] made-drink.
飲ませた is the causative of 飲む (to drink): 'made/had (the baby) drink.' With babies, the causative carries a nurturing sense — 'fed' rather than 'forced to drink.' The に marks the baby as the person being caused to act. ミルク here refers to baby formula or warmed milk, not just any milk.