To me, the main moral of the story is that people must not just accept injustice. Instead, they must fight against things in the world that are evil.
In the book, Luke is a third child in a world where third children are illegal. He has disagreements with Jen (also a third child) over whether they should try to fight back. Jen eventually is killed at a protest against the way that the government treats third children. Luke then comes to realize that he really needs to try to do something to fight back.
That is why I think that is the main moral of the book.
In the book "Among the Hidden" Luke is a young boy who is born to parents who already have two children. The law only allows the citizens to have two children. They put children to death when parents have over three children.
Luke longs to be a part of the real world. His parents hide him while he watches his brothers go off to school where they live normal lives. He has to peak from behind curtains and wonder what the outside and life is like. His parents even have to sneak and grow extra food to share with him because they are only rationed enough for a family of four.
Luke discovers that he is not the only one. He meets another child who is in hiding. However, she and her friends are preparing to take a stand over their situation. When they do they are killed.
What Luke discovers afterwards is that her father was one of the population police, people assigned to monitor and get rid of the extra children. He had kept his daughter hidden and he wants to help Luke. He finds him a family and gets some fake papers so he can become a non-hidden and legitimate person.
The moral of this story for me was that "People have to take a stand against things even if the government makes it a law if it is morally inhumane." I relate this to the Jews who were hidden by the German and others who tried to and succeeded in saving many from the concentration camps and gas chambers. The risks that some people are willing to take on the behalf of others are great, but they live by their moral laws and code not by the laws and code of the government.