PenguinRandomHouse classifies these books as Children's Middle Grade Action & Adventure, and I think that's accurate. This is a fun spin on a post-apocalyptic zombie and monster world with an anti-hero middle grade protagonist. It's probably for the older side of middle grade, kids who won't feel a lot of anxiety about seeing children fending for themselves in that kind of universe.
The author does something interesting to help make that happen. The main character is a foster child who has gone from family to family. His most recent unpalatable foster parents raced out of town without him when zombies and monsters started appearing. Another significant character saw her parents being ferried out of town by authorities, knows they survived the initial attack, knows they didn't want to leave her and may be looking for her. My point being, these kids, at least in this first book, aren't dealing with the pain of finding loved ones among the walking dead. We're not talking a Rot & Ruin-type universe.
Another interesting point about this book--for a middle-grade book it has quite a few of what appear to be illustrations. Except these aren't illustrations. They are graphic elements that are actually carrying part of the story. You treat these things as illustrations and just shoot past them, and you're going to miss some material.
All-in-all, an interesting, entertaining read.