Some links. So far I'm still looking for an activity that would work for a pre-schooler.
White House Kids
Constitution Unit Study. Grades 3-8
Miscellaneous activities for learning about America. Early and upper elementary
Create a Class Constitution. Grades 6-12
Ah-ha! Something for littler ones!
Patriotic Coloring Sheets
Fourth of July Coloring Sheets. Includes the signing of the Consitution