Write a plot line that includes no more than three characters, one of whom---the protagonist---should be a child. A good rule to keep in mind is that children like stories in which it is a child---rather than an adult--brings about a solution to the problem or conflict in the story. The plot line should tell what will happen in the story: for example, "Josh's dog Beansie has disappeared, but a little girl he meets cleverly figures out how to find him."
Create a character profile for each character in the story; you will use these profiles for reference while you write the story. The character profile should list the character's name, sex, age, family situation, personality, a physical description and any other relevant information. For example: Josh is a 10 year old boy, only child of a divorced mother; he is outgoing and has red hair and freckles.
Decide on the setting of your story. Determine whether your story will be set today or whether it will be a historical piece. If the story is to take place in the past, make sure you have done adequate research to portray another period in time realistically. Decide where the story will take place: will it be set in a city, town or the countryside, and more specifically in someone's home, a school---specifically where? Try to limit the places in which your story takes place to no more than two or three locations.
Come up with an intriguing beginning; jump right into the story. Decide on something that will happen---or has just happened--to which the protagonist has a reaction. For example, perhaps a dog can't be found. The first sentence might read as follows: "Josh raced to the end of the block where he heard a dog barking, but the barking dog was not Beansy."
Introduce other characters after the reader has had a chance to become familiar with the protagonist. "It was getting dark. Josh made his way home slowly, dragging the empty leash. A voice called out in the twilight. "You lose your dog?" Josh looked up and saw that the voice had come from a small girl sitting on a porch swing."
Give the characters plenty of opportunity to talk to each other; try to reveal what's happening through their conversation rather than through narration.
Use hints throughout the story so that when the story's ending is revealed, it is a logical solution to the problem or conflict. End the story with a satisfying conclusion rather than a vague idea or cliffhanger.