I’ve done this illustration in front of full grown adults. First, find all of the youngest kids in the audience. Then, after lining them in a row in front of your audience take a tube of toothpaste and put a little bit in each of their hands. After you’ve squirted some in the last ones hand, give him/her the tube and instruct them to put the toothpaste back inside. After letting each of them attempt to put their toothpaste back explain how our words are the same way. No matter how sorry we may be once you’ve said something (even if by accident) you can’t put your words back.