The Enemy by Charlie Higson
When the sickness came, every parent, police officer, politician - every adult - felt ill. The lucky ones died. The others are crazed, confused and hungry. Only children under fourteen remain, and they're fighting to survive. Now there are rumours of a safe place to hide. And so a gang of children begin their quest across London, where all through the city - down alleyways, in deserted houses, underground - the grown ups lie in wait. But can they make it there - alive? The first incredibly gripping adventure in a heart-stopping new series. Warning: Contains strong language and scenes of violence.