Another child starts the story with a sentence and it is written down. Everyone gets a turn until the story is over. Make copies of the text for all the students and have the groups exchange stories so that the children illustrate a story they did NOT write. See illustrators how sees the story differently from the writers. Dreams are fertile places for metaphors.
Drawing is no great trick.
Often illustrators use models or trace photographs they've taken for that purpose. Find a photo with one or two large images of people or animals. Place tracing paper over it and trace just the outline. Now to make a cartoon, do it again, adding your own touches such as patterns on the clothes, jewelry, or exaggerated features.