By now your kid is good at expressing her feelings and emotions. When your kid starts understanding what she is talking about and how she feels on a particular event, she might feel the need to pen her thoughts down.
What you need to know
Before your kid asks you to help, you can volunter to be her mentor and assist her while she writes stories. While she sits to write, you too accompany her by sitting with a sheet of paper. Ask her to select a topic of her choice and both of you write on it. Praise your kid on her write ups. Once your kid is done with writing the story, you can ask her some questions like, 'What made her to write this story?' or ask her to replacing the lead character of the story with another character.