Let everyone in the family brainstorm for ideas, and write all the ideas on individual cards. Each writer might want their own box as well, for ideas they don't want to share. The best way to create ideas is to start with what-if questions. Once you get them started, most kids can think of hundreds of fantasies or problems. Once your children know how to do this well, they'll have a better idea of what constitutes an idea.
Every story needs a character.
Some kids will make themselves the star of every story, and that's okay at first, or once in a while, but eventually they need to learn how to make up characters. Teach older children that authors use elements of real people in their stories, but they practically never use all of a real person in their stories. For one thing, they would get sued. For another, no real person is right for a story, because story people have to be more believable then real people. Look through your books and write short summaries of the characters you encounter.