Here is a little demo video on Vimeo explaining how the app works. It is simple to use.
And here are the very first movies made by my 4 and 7 year old sons, with minimal input from me. They are very short and obviously, there is room for improvement in set design and camera use.
My sons favourite part of this app was the ability to add music and comic captions to their movies.
Have you tried stop-motion with your children?