Write the following words on the board: (the italics indicate how to lead)
Forte-very large movements
Piano-very small movements
Staccato-sharp, robot like leading
Legato-very wavy and flowy leading
A capella-cut off motion to pianist, then lead normally.
Choose a different way to lead for each song (or several diff. ways for one song). See if the primary children can guess how they are supposed to sing simply by watching you lead. Using multiple dynamics for one song takes lots of practice and good eye contact and that's just what we want, right?? :)