Storyjumper is a way of helping your students come up with the inspiration to write a story for children.  The stories can be written and published online easily and once again it is free to use.  If you wish you can also sign up to the classroom edition which allows you to set up a class and manage/review their work.

Here is a link to Storyjumper and below is a tutorial on how to begin using it.  A simple search online will find other similar tools such as Storybird and Zooburst.
Example of use:

Our English department have used Storyjumper as a way of prompting students to write their own stories.  It has proven a simple and successful way to provide the inspiration needed to do this.