Of course if you have an Elements of Art packet (see my store!), you need a Principles of Design packet as well. The Principles are not as easy for students to understand as the elements, but I think this packet is pretty straightforward.
This six page packet gives little exercises for each of the Principles of Design: Balance, Contrast, Emphasis, Pattern, Rhythm, and Unity. (no exercise for movement). Most of these are simple exercises art teachers have been doing forever.
The Principles of Design packet would not be too difficult for fourth and fifth graders to work on independently as a refresher, early finishers, for a substitute, or an end of year quiz!