Liam's Promise by Karyn Langhorne Folan

“This is for real. Get your sister and get to the Mountain Place.”

With those last words from his mother, fifteen year old Liam Harper and his ten year old sister, Lilly embark on a perilous journey of survival.

The bombs are coming any minute—and after they fall, Washington DC and the nice suburb that Liam has called home—will be gone. With only minutes to escape, Liam and Lilly and six other kids—some friends, some foes—embark on a desperate mission to survive first the blast itself, and then the terrifying days ahead in the ruins of the world they once knew. In the wreckage of their community, without food or transportation, their only hope of safety is to walk to a mountain cabin almost two hundred miles away. If they can get to the Mountain Place, they hope to be reunited with Liam's father, a decorated military veteran and survivalist who will protect and shelter them. But the journey brings Liam and his friends face to face with death and danger, deprivation and disease and worst of all: the realization that life will never be the same again.
In this the first of The Doomsday Kids series, Liam and the others confront their losses and find family and hope in each other