Do you remember those days when you were a child (or you had a child or were near a child) who was about four-years-old?
"Let's go to the store," you might say.
"Because we need food."
"Because people need to eat."
"Because if we don't we'll die."
You get the picture. Sometimes the child just asks to be annoying, but many times they ask because the world is so new that they truly don't (or just partially don't) really understand.
After you write your story, ask yourself some of these questions. Pretend that you are being interviewed by a four-year-old. You may run into some answers that are less than obvious or some details that are obvious to you but not to others.