Amongst the Hidden by Margaret Peterson Haddix
To the outside world, Luke does not exist. His parents hide their third son away from a society where the penalty for breaking the Population Law is harsh. When the new housing development replaces the woods next to his family's farm, Luke is not even allowed outside. Sitting alone in the attic, watching the houses opposite, he suddenly sees a girl's face in the window of a house where he knows two children already live.