Start with high-confidence mindfulness for kids books, then narrow by subgenre or adjacent topics.
The Whole-brain Child
Daniel J. Siegel M.D., Tina Payne Bryson
Thriving With ADHD Workbook for Kids
Kelli Miller LCSW MSW, Kelli Miller
A Little SPOT of Emotion 8 Book Box Set (books 1-8: Anger, Anxiety, Peaceful, Happiness, Sadness, Confidence, Love, & Scribble Emotion)
Diane Alber