When designing your children's coloring book, it's crucial to keep the target age group in mind. Younger children, typically under the age of 6, may prefer simpler illustrations with bold, chunky lines and larger spaces to color. Older children, aged 7-12, may enjoy more intricate designs, as well as the opportunity to experiment with different coloring techniques and media. Ensure that the content, complexity, and language used throughout the book are suitable for the age range you're catering to.