"I only use three primaries, so the nice thing is that I can't have favorite colors." -- Chuck Close
I absolutely love this video by Jeff Turick for Sesame Street! It's a great introduction to color for a children's art class, though trust me, I am going to show it to my college students too. Take a moment and then pull out your tempera paint and prove to yourself that yellow and blue make green.
If you are up for the challenge, try this one I gave to my 7-9 year old students last year. With the addition of white for tinting and black for shading, they each came up with a palette of 100 colors.
Draw a grid of 100 2" squares, 10 across and 10 down. Painting within the lines is optional.