。
She put the baby to bed.
Literal
She [topic-は] baby [object-を] bed [in-に] laid-down.
寝かせる (ねかせる) is the causative form of 寝る (to sleep/lie down): 'to lay down' or 'to put to sleep.' When used with babies, it means settling them into bed. The に marks the destination (ベッド). This is the permissive/nurturing reading of the causative — 'helped to sleep' rather than 'forced to sleep.'