Be careful not to write a story that's too short. By trying to keep the story short, you risk leaving out important facts or details or explanations that may be necessary for the child to understand the idea and the moral of your story. On the other hand, don't make it too long. Don't devote too many words to secondary information, as it will only distract your child and overload him or her.
If you're afraid you might get carried away while writing, or on the contrary – forget something, you can use storytelling/writing apps to help you. For example, StoryTold
can help you stick to a story length that is best to your child's age and behavior. Just start writing, we'll take it from there.