Margaret Peterson Haddix
Jonah and Katherine come face to face with Albert Einstein in the fifth book of the New York Times bestselling The Missing series. Jonah and Katherine are accustomed to traveling through time, but when learn they next have to return Albert Einstein's daughter to history, they think it's a joke--they've only heard of his sons. But it turns out that Albert Einstein really did have a daughter, Lieserl, whose 1902 birth and subsequent disappearance was shrouded in mystery. Lieserl was presumed to have died of scarlet fever as an infant.
Imagine a society where only two kids are allowed to a family because of government population control. Luke is a third child, an illegal child, and he has to remain hidden. One day he discovers another third child, Jen, when he sees her face at a neighboring window.