It only makes sense to highlight a coloring book with dogs as the subject. Humans have a strong connection to dogs.
At this point in children’s lives, dogs are more than just animals; they are children’s best friends.
This means that they may be coloring a dog in a coloring book, but to them, they are doing a friend’s portrait.
There is no better way to gain their interest and keep it focused on the subject matter than to include dogs.