Kate Fall has a great post about how to create a successful "unreliable narrator" in middle grade fiction on the MiG Writers Blog. An excerpt:
The trick is to present both what is really going on and what the character is thinking, and to have some reason why the character sees the world as it isn't. I think if you can pull that off, you can have a successfully unreliable middle grade narrator.
Find out more by reading the full post.