。
She'll probably become beautiful when she grows up.
Literal
She [topic-は] grows-up [if-すれば] beauty will-become [probably-でしょう].
成長すれば uses the ~ば conditional: 'if she grows up.' 美人になるでしょう means 'will probably become a beauty' — でしょう expresses probability or gentle prediction. 美人 (びじん) means 'beautiful person/beauty,' a noun. The sentence makes a prediction about future appearance based on current potential.