。
What a kind girl she is!
Literal
She [topic-は] what kind girl [I wonder/exclamation-だろう].
親切な少女だろう uses the same exclamatory なんと…だろう frame, but with a noun-phrase predicate (親切な少女, 'a kind girl'). The な-adjective 親切 attaches the attributive な before the noun 少女 ('young girl'). 少女 (しょうじょ) is itself a slightly elevated, literary word for a young girl — more textbook or narrative than the everyday 女の子. The combination gives the sentence the air of admiring narration, perhaps in a story or a moral lesson — the kind of phrasing you might find in didactic children's literature or a translated fairy tale.