The motivation of children is not easy to achieve. There are several aspects you should check to know if you can encourage kids to write short stories or do any other involved tasks.
Have a good bond with the child. If the child is not comfortable with you, whether as a parent, mentor or coach, he or she will be unwilling to let his or her guard down in order to approach the task. Without establishing a non-threatening environment, your effort, and their motivation, will be lost.
Be patient as you move through your instructions. There is a lot to grasp if this is the first time. You are being tested as well as the child. Keep your words and your pace slow and easy. Do you like having a new task rattled off to you?
When you see a blank stare, be persistent. Can we all get tough stuff on the first try? Be prepared to explain it all over again. To describe it in a different way. To step into their shoes if you must. If they are open to the message, you just have to put it into better terms.
And every kid wants things to be exciting, so make it enticing. What is their motivation if there is nothing to gain. All children want the reward that waits at the end of the line. In fact, for children, reward achievement far outweighs the knowledge that they did a good job, and certainly more than just pleasing you. So if you cannot pull them away from the notion of what they will get in return, you have a lot more enticing to do.
When they complain that they don't get it, be understanding. If there is ever a time that they are ready to react poorly to your mood, this is it. If you react sternly, you will lose them for sure. Take a deep breath... before you start anything... and understand that they missed something because they didn't know what you meant.
And finally... ask lots of questions. If you got this far, then you can now bring it home. The hard part is done. They produced something. Now, by having them answer your questions, you show that you are interested and they will eat that up. It is it's own reward, and hopefully what you promised will be less important.
So, in short, remember... Bonding, Patience, Persistence, Enticing, Understanding and Questioning.