。
She gently put the child to sleep.
Literal
She [topic-は] child [object-を] gently put-to-sleep.
そっと is a soft, atmospheric adverb meaning 'gently, quietly, carefully' — the kind of action you take when not wanting to disturb someone. 寝かせる is the causative of 寝る ('sleep'), but with young children, the causative reads as gentle care rather than coercion: 'put to sleep,' not 'made to sleep.' Japanese causatives carry that tonal range — coercive ('made') or permissive/caring ('let' or 'put') — and context decides which one lands.