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.

"The golden age" of coloring books is believed to be 1960-s, because almost all characters were depicted in them, as well as some products (like Planter Peanuts or Campbell Soup)

Kids Coloring Book Pages