Am I the only one who didn't know Audrey Hepburn & Julia Childs were secret Agents in WWII?

No because they weren't secret agents. Child's was a research assistant for the OSS during the war. Hepburn lived in Arnhem, Netherlands during the war and studied and worked as a ballerina. She was in Arnhem during the famous Market Garden battle in 1944, but neither one of them was a secret agent.