The middle grade me would give it a 5 star but the adult me would only give it a 3. 4 is a good midpoint ;).

I like the characters and the story. I also like the narrator... insisting that the secret is bad. Of course that would make any curious person want to open the next page.
What I'm not too crazy about is the ending. By that time the secret thing had lost its magic for me.

All in all, it's a very enjoyable story. Highly recommended for middle grade adventure lovers!