It is believed that the company The McLoughlin Brothers (en.wikipedia.org/wiki/McLoughlin_Brothers) invented the coloring books. They published a book called The Little Folks' Painting Book in collaboration with Kate Greenaway in 1880.

Basically coloring designed for children from 4 years. It is necessary to offer the child a few options for the different colorings. It happens that some of them are out of child’s interest, while others will love it. So try to pick up the coloring in accordance with the interests of your children.

Alphabet To Color