People often asked: "Why is it that I can't speak German?
"The simple answer is my mother refused to teach me." She "did not want to reflect on her history", Mr Key said.
The prime minister also remembered times his mother was "crying in the corner" after being confronted with news stories recounting atrocities from World War 2.
Several Holocaust survivors also spoke at the memorial.
Moshe Orgad, a Serbian Jew, was adopted by a non-Jewish family when Belgrade fell under German control.
His mother's decision to flee the city divided his family.
His grandfather argued that Germans would never harm the Jews, Mr Orgad said.
Despite this, he was sent to live with "Sava" and his family. He was eventually reunited with his parents, whom he barely recognised, at the war's end.
"I was lucky to be saved from the Germans by Sava and not to be deported to Auschwitz and other extermination camps."