Materials: magic marker, balloon, paper clip, some tape and a paper bag.

First you need to blow up the balloon and write a name on it. Then put a few pieces of tape on the back so the children can not see it. Then unfold the paper clip and ask the children to say a few nice things to the person. After each nice word poke the tape. After they have said a few nice words get them to say a mean word and when they say something mean do not poke the tape poke the balloon so it pops. Then tell the children how saying mean things to other people is making them feel bad and sad.
Then read Luke 6:27-31.
When I did this in my church even the adults liked it and thought it was a very good message.