Everland (the Everland Trilogy, Book 1) by Wendy Spinale

Forget the story of Peter Pan and Wendy you know. Because in Everland, the only way to grow up is to survive.

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 younger siblings, Joanna and Mikey. They spend their nights scavenging and their days avoiding the deadly Marauders-the German Army led by the cutthroat Captain Hanz Otto Oswald Kretschmer.

Unsure if the outbreak has spread but desperate to leave, Captain Hook is on the hunt for a cure, which he thinks can be found in one of the surviving children. He and his Marauders stalk the streets snatching children for experimentation. None ever return.

Until one day when they grab Joanna. Gwen will stop at nothing to get her sister back, but as she sets out, she crosses paths with a daredevil named Pete. Pete offers the assistance of his gang of Lost Boys and the fierce sharpshooter Bella. But in a place where help has a steep price and every promise is bound by blood, it might cost Gwen more than she bargained for.