My favorites:

Yoga and Mindfulness books for kids

Peaceful Piggy Meditation by Kerry Lee MacLean

The Listening Walk by Paul Showers

A Quiet Place by Douglas Wood

Yoga, Mindfulness and Mediation books for adults:

Mindfulness Skills for Kids and Teens by Debra Burdick

Calm and Compassionate Children by Susan Usha Dermond

Relax Kids- The Wishing Star (52 meditations for children) by Marneta Viegas