Claire was born in Debrecen, Hungary, in 1932. She lived with her parents, Rose and Joseph Grunwald, and her five sisters: Silvia, Brindle, Goldie, Dino, and Chani. Her family ran a grocery store, and her mother operated a butcher in their kitchen.
Debrecen had a vibrant Orthodox Jewish community, and Claire attended a Jewish school. The city had ~10,000 Jews, with many synagogues and Jewish educational institutions.