Can Any Educators Recommended Some Children's Mindfulness Books/Activites?

Was wondering if there are any educators here that could recommended some age appropriate contemplative-style books or activities that deal with Mindfulness or related topics? 

Preferably looking for books/activities appropriate for first-graders. However my interest in Contemplative Education is broad (hopefully going to graduate school for it), so any books/ activities that ya'll have found interesting or exciting in this area I'd love to hear about it!