As an elem teacher of gifted readers, you have LOTS of great suggestions here.
I would add: Charlotte's Web, The Secret Garden, Matilda, Ramona Quimby and Stuart Little.
http://www.teachersfirst.com/100books.cfm
My kids loved the Magic Tree House series, but also lost interest in Narnia as, although short, it has a lot of "old-time" language in it that was confusing. Be careful also with some of the classics for they too, unfortunately, sometimes give the younger kids trouble and are better for "read-alouds" than "read-alones."