Shadow Children are third children who are born illegally. These children, when caught, are commenly being killed or imprisoned as punishment.
The Population Law was made when a great number of droughts and great loss of food hit a unnamed land (supposedly the united states), forcing many of the nation's population into poverty. The Population Law stated that only two children were allowed per family. Any child born after the initial two would be inprisoned or killed, along with their family or whoever else was caught bearing them. If a mother became pregnant for a third time, they were required by law to have an abortion.
A "Shadow Child" is a child born after the first two children, forced to hide in their own homes and living in constant fear of being discovered. Their families usually keep them hidden for at least until they are fully grown or until they can obtain a fake I.D.
Luke Garner is the first shadow child introduced in the first book, Among The Hidden
Jen Talbot was the only named third child killed