。
She put her child to sleep.
Literal
She [topic-は] child [object-を] put-to-sleep-tucked-in.
寝かしつける is a multi-layered compound: 寝る ('sleep') in causative form 寝かす ('put to sleep') + つける ('attach, fix') — the つける suffix here adds 'completing the action / making it stick.' Together: 'put to sleep [until they're properly down].' This is the verb you use for tucking a child into bed and getting them to actually fall asleep — not just laying them down. A single compact word covers what English splits into 'tuck in,' 'put to bed,' and 'get to sleep.'