Learn the basic scientific concepts involved in the changing colors of leaves in the fall so you don't include any confusing or misleading information. Determine the essential facts you want to convey. For instance, list the facts that leaves change from green to shades of red, yellow and orange due to the loss of chlorophyll, that chlorophyll uses the sun's energy to make food for the tree during summer, and that in the fall the tree prepares for winter by stopping chlorophyll production. Build the story around this basic information.
Create characters for your story. For instance, make the main character the tree itself or an individual leaf, or present the pigments as characters with the chlorophyll character "disappearing" until spring. Give the children characters with whom they can identify rather than as abstract concepts to make the information easier for them to understand.
Begin the story at the height of summer when the days are long, the leaves are green and there is plenty of sunshine. Introduce your characters. If they are leaves or trees, they will be very green at this point; if they are pigments, the chlorophyll character will be present. Create a very basic simple storyline for very small children; for older children you can make the narrative more complex.
Move to the middle section of the story by describing how the season changes from summer to fall. Explain that the nights grow longer, the days grow shorter and the weather grows cooler. Relate this to changes in the colors of the leaves; because there is less sunshine, the tree or leaf character stops wanting to produce chlorophyll. Use repetition and rhythmic language to enhance children's enjoyment of the story as the narrative progresses. For example, repeat a sentence like "And the nights grew colder and the days grew shorter," containing two identically structured clauses and a regular stress pattern. Children enjoy listening for repeated phrases and reciting them along with you.
Move toward the end of the story by revealing that the leaves have now turned color. Leaf or tree characters will now be shades of red, yellow, orange and gold; chlorophyll characters will now have disappeared until the following spring, leaving only anthocyanin and carotenoid characters. Continue to engage the children's attention by encouraging them to take part in the telling of the story. For instance, ask questions such as "And what color do you think the leaves are now?" so that children can supply the answers when they are having the story read aloud to them. Alliterative phrases like "rich russet red" can also help make the story enjoyable and memorable for children.
Copy your story on paper, either print clearly by hand using a black pen, or type it up on a computer using a large font size. Gather sheets of paper together to make a booklet, writing a few lines of the story on each page, picture book-style, so that you can read through it with children. Illustrate the story with pictures of leaves, trees or pigments, using photos, paints, crayons or magic markers.