Ask the children to draw a person on a sheet. A stick man is enough.
Ask the children to say hurtful things to that paper person.
Tell them that with each offensive statement, they must crumple a portion of the paper on which that person is drawn until the person is completely in a ball.
Now say nice things about this person. With each nice compliment, smooth the paper as best you can.
The end result is still a little crumpled. The idea is that hurtful words hurt, even after saying beautiful things. It is a principle to teach all children. You could also demonstrate the concept without giving a paper to each child.