。
She must have been a cute girl when she was young.
Literal
She [topic-は] young days [contrast-は] cute girl was [must be-に違いない].
かわいい女の子 (cute girl) uses the attributive adjective かわいい modifying 女の子 (girl). に違いない expresses confident inference about the past based on what the speaker observes now. かわいい has a warmer, more approachable nuance than 美しい (beautiful), suggesting charm and endearing qualities rather than formal or striking beauty.