I’ve just been recommended this resource, Be the Boss of Your Pain. 
It teaches children to manage and relieve the pain of common headaches and stomach aches, as well as pain related to injuries and illness.
The book covers self-care skills including belly breathing, positive self-talk and aromatherapy.
It’s in a softcover book, 64 pages, with color illustrations and is suitable for for ages 8 & up. It costs about $8.95 (US).
There’s a couple of other interesting titles in the series. You can check them out at Free Spirit Publishing.