。
She behaved like a young girl.
Literal
She [topic-は] young-girl [like-のように] behaved.
小娘 (こむすめ) means 'young girl, slip of a girl' — it can carry a condescending tone depending on context, suggesting immaturity or naivety. ~のように ('like, as if') marks the simile, and 振る舞う (ふるまう, 'to behave, to conduct oneself') is a somewhat formal verb for behavior.