。
She left the child with her aunt.
Literal
She [topic-は] child [object-を] aunt [to-に] left-in-care.
預ける is the everyday verb for leaving something or someone temporarily in another's care — a coat at a coat check, money in a bank, a child with grandparents. Lighter and more transactional than 委ねる ('entrust'), but the same に marks the recipient. The kanji 叔母 specifies a younger aunt (parent's younger sister); 伯母 (older aunt) shares the same reading おば but is reserved for elder relatives, with the distinction visible only in writing.