Here are some great fiction series for kids, organized by early elementary, tween/middle school:

Early elementary

Ivy and Bean, by Annie Barrows

Katie Kazoo, by Nancy Krulik (1st-3rd grade)

Magic Tree House, by Mary Pope Osborne

Mr. Putter and Tabby, by Cynthia Rylant (1st-2nd grade)

My Weird School and My Weird School Daze, by Jim Paillot

Tween/middle school

Cirque du Freak, by Darren Shan (tween/middle school)

Conspiracy 365, by Gabrielle Lord

Dear Dumb Diary, by Jim Benton (tween/middle school)

The Land of Stories, by Chris Colfer (tween/middle school)

Ranger’s Apprentice, by John Flanagan

The Secret Series, by Pseudonymous Bosch

Series of Unfortunate Events, by Lemony Snicket

Suddenly Supernatural, by Elizabeth Cody Kimmel

Zeke Meeks, by D.L. Green