This technique works really well with picture prompts, so I found some pictures on Wikimedia Commons that might suggest a story idea, and accompanied each picture with questions I brainstormed. I'm sure you and your kids will come up with more. Ignore the questions that don't spark anything, and run with those that do.
Some questions will lead to more questions. Some will have kids tripping over their tongues in eagerness to express ideas. Kids can respond to question in thoughts, out loud to a partner, out loud and record themselves to listen to later, or try to jot down ideas via the computer or pen. The questions can even be answered in whole sentences, and used to build up character profiles or plot outlines.
Where might this be? Does the place have a name? What sort of place is it? (Setting) What sort of creatures are in the tree? Do they have names? How did they get there? What are they doing? What are they thinking? What are they saying? If you were there, what might you be able to hear/feel/smell? What is the sphere on the ground? How did it get there? What will happen next?
What is this? What does it smell like/taste like/feel like? How did it get here? What properties does it have? Can it change something or cause something? What is it called? Does everybody call it the same name? Who uses it and what do they do with it?
What is this? Where is it? Where does it lead? Who lives on the other side?
What's happening? Who is the man? What's his name? Why is he here? Where is he? How does he feel? What can he smell/hear in this place? What will happen next?
If you'd like more ideas about encouraging children to write, click on Writing in the right sidebar, or check out How to Encourage Kids to Write, or Fast and Fun Writing with Kids.