You can start by having them read a lot of stories to get familiar with different structures and styles. Then, encourage them to come up with their own ideas and talk about them.
One way is to read short stories together and talk about the elements like characters and plot. Encourage them to imitate those in their own writing. Also, give them prompts or topics to get them started.
First, think of simple and interesting ideas like a magical adventure or a day in the life of a cute animal. Use easy words that kids can understand. For example, 'big' instead of 'enormous'. Then, create characters that are relatable to kids, such as a little boy or girl, or a friendly monster. You can also add some colorful pictures if possible to make the story more attractive.
Once upon a time, there was a kid named Tim in school. He was a bit different, wearing thick glasses and being really into books. Some of the popular kids in the class started to bully him. They would knock his books out of his hands in the hallway and call him names like 'nerd' and 'four - eyes'. Tim was really sad but he didn't tell anyone at first. One day, a new teacher noticed what was going on. She started to keep a closer eye on those bullies and also encouraged Tim to be proud of who he was. Eventually, with the teacher's help, the bullying stopped and Tim became more confident.