。
She'll probably put the doll to bed.
Literal
She [topic-は] that doll [object-を] bed [on-に] will-lay-down probably.
寝かせる is the causative of 寝る ('sleep'), but lexicalized as a verb in its own right meaning 'to lay (something/someone) down to sleep, put to bed.' This use is especially natural with babies, pets, and — as here — dolls treated as such. The に marks the destination ('onto the bed'). ~だろう adds speaker conjecture about a future action: 'she'll probably do this.' The whole sentence captures how children — or adults playing with dolls — extend the rituals of childcare onto an object, treating it as something that 'sleeps.'