Ela Stein Weissberger was 11 years old in 1943, when she landed the role of the Cat in the first performance of Brundibar, a children's opera about a gang of kids who take on a greedy organ-grinder. While not a glamorous production, it resonated deeply with its audience, the prisoners of the Theresienstadt concentration camp in Czechoslovakia. "People loved to come and sing along," recalls Weissberger, who was in the camp for three years. "Especially the victory song."
Five years ago, Maurice Sendak came across the long-buried opera. He was electrified. "It's a very simple story, the most basic of...