One of our members came across this great blog http://www.klsriley.com/. Here, you can see lesson plans and files to help your students compose pentatonic pieces. She has samples from her second graders there! I plan on using this later next month with my little ones.
Here is one of her plans: http://www.klsriley.com/2013/03/07/third-grade-pentatonic-compositions/.
There are downloadable pdf files and smartboad files! This is so helpful for anyone needing a jumping off point for composition.