How does Helen feel about leaving Mark and what is his answer and why does he say it?

Helen Bragg does not want to leave with the children because it makes her feel like a deserter. Mark wants them to leave because they live in a primary target area. She wants to stay with him but does not realize he will be underground in the Hole and she will be on the surface exposed. She believes her job is to stay with him but Mark says her job is to survive and care for the children.