The Last Kids on Earth #3: Last Kids on Earth and the Nightmare King book by Max Brallier

Life after the zombie apocalypse is pretty good for 13-year-old Jack Sullivan: he lives in a mind-clobberingly cool tree fort with his best friends, speeds through town playing Real-Life Mario Kart, has a crew of monster buddies, battles zombies on the regular, and generally treats life like it's a videogame!
But then Jack's friends make a startling discovery: they may not be the last kids on earth, after all. This is great news for everyone... except Jack. Once they've found other humans, his friends won't stick around for long! Jack's only hope for keeping things the way they are is to prove that everything here is perfect, life is crazy fun, and nothing else could be any better.
One problem: it's hard convincing his friends that everything is great when they're being hunted by a monstrous Nightmare King and an ancient evil who won't rest until Earth has been devoured. Crud! Maybe life after the monster apocalypse is more complicated than Jack thought...