The schools should hopefully be able to advise how to go about it. Also set some parameters around the story, like a word limit. And unfortunately, with the prevalence of chat GPT type tools, some kids may be tempted to generated stories and submit as their own work, so you might ask the schools how to identify and deal with that. It would be very unfair to award something to a kid that cheated. But it does sound like a fun Idea to get kids interested in writing.