Willow Run by Patricia Reilly Giff

The year is 1944 and we are at war. Margaret lives with her parents, her brother has gone off to war. One day Margaret catches some neighborhood boys painting a swatika on her Grandfather's house and she confronts them. She tells them that her grandfather is not a Nazis even though she knows he is quite German. They tell her if this was anywhere but Rockaway, NY her Grandfather would be put away in jail or worse.
This begins Margaret's concern for her Grandfather and she is worried that something will happen to him. In fact when it comes to the family moving for awhile because of her father getting work in Willow Run, Michigan, she is very against her Grandfather coming with them. She is positive that her Grandfather will be better off right where he is.
As time goes by things happen and it becomes necessary for the family to be together. Margaret writes a secret letter asking Grandfather to come and stay with them. In the end Margaret is very proud of her Grandfather and her is a great comfort to the family as they wait for word from her brother who is MIA.