They can both mean "girl", but "بنت" also means "daughter / daughter of ..." . Originally, the word "بنت" was used for little girls, while "فتاة" was used for girls after puberty, but in modern times most people don't make those distinctions, and the word "فتاة" isn't used in everyday language in most Arab regions, the word "بنت" is used for girls in general.

ولد وفتى بنت وفتاة مترادفات

There is no different at all.. They has the same meaning