Preparation. Make a gumball poster with the title "Chews" the Right at the top. Poster board would work well for this if you are planning on using it more than one week. Make a paper circle gumball for each song to be sung in the Primary Program. Use bright colored paper. Grab a large, clear plastic container for your gumball machine and a big bag of gumballs. You can buy these are party stores in a variety of colors.
Execution. A child picks a paper gumball and the group sings the song. A teacher judges how well they sing by your criteria (know the words, smile, stand up/sit down together, sing out, etc.) At the end of the song the teacher-judge decides how many gumballs go in the gumball machine (plastic container.) Give the judges a limit so they don't dump in all the gumballs on the first song!
Re-do's. Don't be afraid to redo songs. Make them really work for their gumballs and they will rise to the occasion. If you notice they didn't start the song off strong, stop them and say "We can do better than that!" and let them show you. If you notice some kids goofing off, stop the song and remind them what a great choir they are and how great they sound when everyone is singing. Don't spend too much time talking, let them show you how well they can sing.
Bonuses. Let them earn bonus gumballs for things like singing it without your voice, or the teachers' voices. Or singing it with dynamics. Being quiet between songs. Singing out their testimonies. Doing the signs. Praise them for the little things you notice and they will want to please you. They just need to know what your expectations are.
Reward. The fuller the gumball machine gets, the more gumballs they all get when we are done with the program. I'll divide up the loot and hand it out after the program, a baggie for each class. Plastic gloves will come in handy when handling all those gumballs!
Have fun "Chews"-ing the Right!