。
She held the child tightly to her chest.
Literal
She [topic-は] that child [object-を] chest [target-に] hugged-tight.
抱きしめる is a compound verb fusing 抱く ('embrace, hold') + 締める ('tighten, fasten'), giving the meaning 'to hold tightly, embrace firmly.' 胸に ('to the chest') marks the target of the holding — pulling the child against her body. This kind of physical-affection phrasing is more often associated with maternal or paternal embraces in Japanese narrative; it's also common in emotional reunion scenes, comforting scenes, or moments of fierce protectiveness.