Get to know your readership. Don't just go on what you know about children--talk to them. Visit a school, library or children's play center and ask the kids what they want to hear about in a story. Get an idea about their imaginations, what they already know about turkeys and Thanksgiving and what their favorite books are. This way, you'll have an idea about what other books have succeeded and what sorts of story details you may consider including.
Write the outline of your story. Since the turkey story is for first-graders, the plot and characters must be appropriate for young readers at that level. According to the Writing World website, writers for young children should keep their stories short. This means you'll have to build your story's world in less space. Consider some elements or motifs of a turkey or Thanksgiving story, such as generosity, sharing or kindness. Think of some characters that might help a child understand Thanksgiving---a family sharing a turkey dinner or a friend giving another friend without food some of her own dinner.
Write your story using clear, short words that a first-grader can understand. Make sure each word you use needs to be there and that your sentences tell a coherent story. According to Writing-World, children's writers should tell vivid, imaginative stories that capture the reader while also paying attention to detail. Limit your use of passive voice and use actionable verbs. For instance, don't write "Tommy was climbing the tree" when you could write "Tommy climbed the tree." Make sure your story is fun and slightly challenging but avoid long words that aren't appropriate for first-graders.