Extinction Effect - Robert Wright - Bog

Outside, that's where the danger lay, not with some child. Not that any zombie could get through the razor wire surrounding the camp, but the captain wasn't one to take chances. Besides, there might be a few aliens left from the unit they had destroyed. - from Extinction Effect: Zombie Run

In a land overrun with zombies, aliens, and killer rabbits, Tanya finds herself in the middle of a child army. All the while her friend, Bo Jangles, scours the land looking for her.
This second book in the Extinction Effect series, follows the continuing adventures of Tanya and Bo Jangles as they struggle to survive in their new reality.