This has quite a few flaws. It definitely feels like a first novel, and could've benefited from more editing. Too many plot elements, not tight enough. The author is a screenwriter and it shows; his dialogue is good and snappy, but the rest of it needs work. Pretty derivative (or perhaps paying homage?) in many places, hard to follow (I still don't understand all of the time travel 100%), and fully of so. much. telling rather than showing. I don't know why dwarves from (granted, magical) North America speak with a Cockney accent? Scottish brogue?
That said, it's also charming and enjoyable and reasonably exciting, and while the time travel plot is hard to follow, it's interesting at least. I think I could get kids to try this one. The emotional subplot is pretty affecting and well-done, even though the emotional descriptions themselves can get tedious. The villain is good too (although villains are pretty easy to do). A little long (again, more editing pls!), but nothing a Harry Potter fan can't handle.