I did this with the exact same age group. Like some of the others said, I provided 6" cakes for each kid. I iced each cake smooth and put it on a board. Then I made several colors, a couple bags of each color, with a coupler on each bag. I traced a couple of patterns of flowers and a heart on paper (you could also use cookie cutters) and let the girls use toothpicks to trace around the patterns, so they had a template to trace over. I showed them how to hold the bag, how to do a line, basic shells, etc, and just let them go to town. They had so much fun and it was pretty easy to set up, clean up, etc. They also loved it because they were able to take something home that was already completely decorated and it was so much more satisfying to complete something.
Most of all, have fun with it!