Here are few ways you can get started with stopmotion concepts.
ABCya! Animation for Kids
Go to ABCya! Animation and see if you can make the ball bounce.
Pivot Stick Figure
Explore stopmotion techniques with Pivot Stick Figure, which is a free software program that allows to create simple animated image files frame-by-frame. (You can download Pivot here. If you have a Mac, or if you want a more advanced program, you can also download the free Stykz software).
See a sample collection of Pivot dance party movies made by sixth graders.