Two illustrated nonfiction books that will explain leap year are The Leap Year Book by Barbara Sutton-Smith
and What in the world is a Leap Year?: And Other Time Related Measurements by Desiree Bussiere.
I have also found several fiction books about characters born on February 29th that include an explanation of why their birthday is only on the calendar every four years. Two that you and your students might enjoy are It's My Birthday Finally! by Michelle Whitaker Winfrey,