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.
Today there is a large set of coloring books. You can find coloring-cursive books, books outliners, for boys (all sorts of appliances, machines, robots, etc.), for girls (fairy-tale characters, dolls, funny funny little animals). There's even a wallpaper coloring books that opens a wide field to have enough space for young artist.