What about The City of Ember books? I've read them recently, and there's nothing in them that wouldn't be appropriate for a younger kid. The protagonists are a little older (14, I think) but they're pretty tame.
Also, just to throw this out there: Scholastic.com has a book finder tool where you can put in a book and find similar books and skew the reading level. So you could actually (I think) put in Magic Treehouse and pull the reading level slider up a bit.