Children are naturally attracted to animals, especially familiar ones, like local farm animals, or pets. This natural attraction gets their attention, and a writer would be able to piggy-back another subject or topic within the coloring book. For example, The Three Little Pigs could lead into the subject of houses, and how they are built, and maybe, about why the should be well built. Children can relate to most any subject if it is brought to them in a colorful manner.