Having children write an autobiographical or semi-autobiographical story is a great way for them to start with creative writing. Encourage a child to write about something interesting that he did recently. Alternatively, you could suggest writing a story where the child is the main character. The story could start out normally, with something that really happened, and then he could describe some imaginary events from there.
Another interesting creative writing idea is to get children to write a transformation of an existing story by changing the point of view that the story is told from. Get the children to select a familiar or well-liked story and turn it around. For instance, they could retell "Little Red Riding Hood" from the perspective of the wolf. Suggest that they empathize with the new protagonist, even if it's a bad guy.
Sometimes children need just a little bit of inspiration to get them started on their creative writing. Come up with a title that you think will interest the writers, then present them with the title and get them to write a short story starting from there. This can be particularly interesting with a group of children, because you can compare the different interpretations of the given title.
Another way to inspire children to write a creative piece is to give them a genre to work from. This works particularly well if they've been studying a genre in school or have been reading it at home. For instance, you could tell children to write a ghost story, a fantasy story, a detective story, a mystery or an adventure. Make sure they understand the expectations of the genre, and let their creativity run wild from there.