Describing the separations in her 2017 testimony at Queens College, Ms. Geulen spoke of how hard it was “to tear a child away from his mother and not tell her where we were taking him, and to have her cry and cry, ‘Tell me at least, only tell me, where you’re going to take him?’”

“If I’d had children then, I don’t know that I could have done it,” she said.

Each child was given a new name: Sarah became Suzanne, Moses became Marcel. But young children often didn’t understand what was wrong with their real names, or why they couldn’t tell strangers that they were Jewish. One time, Ms. Geulen recalled, she was on a train with a girl she was smuggling to safety when another passenger asked the girl her name. The girl turned to Ms. Geulen and asked, “Should I tell her my new name or my real name?” Luckily, the passenger was not sympathetic to the Nazis.

Ms. Geulen, using the code name Claude Fournier, often had to walk great distances to reach hide-outs in rural areas, carrying a suitcase in one arm and a child slung across the opposite hip. “I would have to stop every 30 feet, put the suitcase down and change the child to the other side,” she told Professor Griffin.

Ms. Geulen was favored by her fluency in German and by her blue eyes and shoulder-length blondish hair, giving her the visage of the so-called Aryan woman whom the Nazis idealized. One time she was walking alone on a Brussels sidewalk on her way to pick up two children while carrying their names on a slip of paper hidden under the inner sole of one of her shoes. A street photographer snapped her picture at a moment when a German officer happened to be striding a few steps behind her. Not knowing if she had been set up, Ms. Geulen called her handlers at the committee and was told to find the photographer and get the negative. Perhaps because he was disarmed by her beauty, he gave it to her, Professor Griffin said.

In May 1943, the Germans raided Ms. Geulen’s boarding school (today called the Isabelle Gatti de Gamond Royal Atheneum), where a dozen Jewish children were hidden. The school’s headmistress, Odile Ovart, and her husband were sent to concentration camps, which they did not survive. Ms. Geulen was interrogated but released. When a German officer told her that she should be ashamed of teaching Jewish children, she responded, “Aren’t you ashamed to make war on Jewish children!”