。
She had the child sit on the chair.
Literal
She [topic-は] child [object-を] chair [on-に] made-sit.
座らせる is the causative of 座る ('sit'). With a small child, the natural English reads as 'had her sit' or 'sat her down' rather than the literal 'made her sit' — Japanese causatives span the full range from gentle direction to forced compulsion, with context choosing the tone. Note 椅子 written in hiragana as いす, common in casual prose, children's books, or wherever the kanji feels stiff. The に here marks the destination of the seating motion.