The Enemy (Enemy Series #1) by Charlie Higson

In this first book in a series set in post-apocalyptic London, children under the age of sixteen must battle for survival against grown-ups who succumbed to a disease that turned them into flesh-eating monsters. In the wake of a devastating disease, everyone sixteen and older is either dead or a decomposing, brainless creature with a ravenous appetite for flesh. Teens have barricaded themselves in buildings throughout London and venture outside only when they need to scavenge for food. The group of kids living a Waitrose supermarket is beginning to run out of options. When a mysterious traveler arrives and offers them safe haven at Buckingham Palace, they begin a harrowing journey across London. But their fight is far from over-the threat from within the palace is as real as the one outside it. Full of unexpected twists and quick-thinking heroes, The Enemy is a fast-paced, white-knuckle tale of survival in the face of unimaginable horror.