Enemy series (Charlie Higson)

They'll chase you. They'll rip you open. They'll feed on you ... When the sickness came, every parent, policeman, politician - every adult - fell 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,...

In London, after a worldwide sickness infects adults, turning them into zombie-like creatures, DogNut and his crew embark on a deadly mission to find missing friends, as hungry, bloodthirsty grownups lie in wait.

"First the sickness rotted the adults' minds. Then their bodies. Now they stalk the streets of London, hunting human flesh. The Holloway crew are survivors. They've fought their way across the city and made it to the Natural History Museum alive--just barely. But their fight will never end while the Enemy lives, unless there's another way... The kids at the museum are looking for a cure. All they need are medical supplies.To get them they must venture...

"The final battle ensues between Saint George's army of sickos and the army of London kids"--