Tellagami is a free app for iOS and Android that lets you use a character onscreen to tell a story. You can pick the background and setup your scene. Fairly easy to understand and kids won't need much instruction on how to make it work.
What is additionally nice about it is that you have great character placement tools. The blog iPad Educators has demonstration of Tellagami given by a man with a film background. He shows how easy it is to put the character in correct perspective to the background image.
Have you used Tellagami with students? Reply to this post with an example you or a student made. We'd all love to see it.
Howard