、。
Although she's a child, she can act with good judgment.
Literal
She [topic-は] child [though indeed-ではあるが], discernment-having behavior [subject-が] can-do.
ではある adds emphatic affirmation onto the basic である copula — 'is indeed,' 'while granted X' — and the が that follows pivots into a contrast: 'while indeed X, [yet] Y.' A literary, slightly formal frame for concession. 分別ある is a fixed adjectival phrase: 分別 ('discretion, good sense, mature judgment') + ある yields 'having discernment,' modifying nouns directly: 分別ある人, 分別ある行動. The ability pattern ~ができる takes the action-noun as が-marked subject, neatly pairing here with 分別ある行動.