The Question and Answer section for Schindler’s List is a great resource to ask questions, find answers, and discuss the novel.
As I recall Iyzhak Stern helps Schindler organize everything about his "munitions" factory.
She represents the innocence of the Jews being slaughtered. If you noticed she is shown twice in the movie. The first is when she is walking and hides under the bed. The second time is when she was deceased be carried by the wheel barrel.
He wants them to make faulty shells for the Nazis.