This is a book I can see middle graders enjoying. There's a good mixture of adventure and as soon as we reach Nightmare, there's plenty of quirky characters. Charlie has a good core group of friends that middle grade readers will enjoy.
All that said, I still think we need to caution against stereotypes - we have a nerd with glasses who is failing gym, the immediate perception that the stepmother is a witch, and an athletic friend who is failing his academic classes. These stereotypes need to change and characters need more depth. They're not offensive, but they are disappointing.
While this book wasn't for me, it wasn't the worst either. As I said, I do think the intended audience would really enjoy it. Additionally, there's a decent message woven in that it's easy to become self-absorbed and it hurts the people around you.