。
Though she was just a child, she was brave.
Literal
She [topic-は] child was [but-が] brave was.
A compact sentence in literary register. であった is the writing-style copula past, more elevated than spoken だった — common in biographies, narrative essays, and historical writing. 勇敢 ('brave, courageous') is a Sino-Japanese compound (勇 'courage' + 敢 'daring'), heavier and more formal than the native 勇ましい. The mid-sentence が flips on the contrast — 'was a child, but was brave' — a frame so fundamental to Japanese it appears in everything from fairy tales to modern news copy.