In the grim dystopian fantasy world depicted by Margaret Peterson Haddix in Among the Hidden, families are strictly forbidden by the government from having more than two children. This is to prevent further overpopulation in an already overpopulated society.
If any third children—or shadow children, as they're also known—should be found, then they will be killed. The responsibility for hunting down and killing shadow children has been given to the Population Police, a ruthless, all-powerful organization that strikes fear into the hearts of shadow children and their families.
What's particularly scary about the Population Police is that they operate without any meaningful constraints on their power. They're authorized to do whatever it takes to hunt down and kill shadow children.
But the Population Police have no moral qualms about the work that they do. And in any case, they have the full force of the law on their side. In this society, the very existence of these children is illegal; there's a law against Luke, the protagonist, and every other shadow child in society. That being the case, it's dangerous in the extreme to be a shadow child. All you can do is hide from the Population Police as best you can and hope they won't catch you.