One of the first psychologists who in the early twentieth century used the coloring mandalas as a relaxation technique was Carl Gustav Jung. He prescribed his patients coloring mandalas as part of the therapy.
Try to use a themed coloring. For example, a series of coloring books about animals, buildings or machines give children an idea of the objects of the world. Coloring of fairy tales or cartoon characters, are good because they allow the child to "talk" with their favorite characters and express their attitude towards them through colors.