The younger the child, the larger and easier the drawing is. The outlines are indicated by a thick line from 2 to 4 mm, so that for the child it is easier to stay within the contours.
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.