What is the theme of the book Among the Hidden by Margaret Peterson Haddix?

The conflict in Margaret Peterson Haddix's book "Among the Hidden" is that Luke is the thrid child of his family, and by law each family is suppose to have only two children. As Luke grows up he has to have a different identity, so the conflict is between Luke and his parents because he doesn't want his life to be a lie. While his parents are making sure he doesn't get caught he meets another child that is third in her family and her name is Jen.