Everland (Everland, Book 1)

London has been destroyed in a blitz of bombs and disease. The only ones who have survived are children, among them Gwen Darling and her siblings, Joanna and Mikey. They spend their nights scavenging and their days avoiding the ruthless Marauders -- the German army led by Captain Hanz Otto Oswald Kretschmer.

Unsure if the virus has spread past England's borders but desperate to leave, Captain Hook hunts for a cure, which he thinks can be found in one of the survivors. He and his Marauders stalk the streets snatching children for experimentation. None ever return.
Until the day they grab Joanna. As Gwen sets out to save her, she meets a daredevil boy named Pete. Pete offers the assistance of his gang of Lost Boys and the fierce sharpshooter Bella, who have all been living in a city hidden underground. But in a place where help has a steep price and every promise is bound by blood, it will cost Gwen. And are she, Pete, the Lost Boys, and Bella enough to outsmart Captain Hook?