Sing songs about kindness from the CS (see the Topics index).
Read Matt. 7:12: "all things whatsoever ye would that men should do to you, do ye even so to them: for this is the law and the prophets." H
elp the children distinguish between "doing what has been done to them" ("an eye for an eye"--the Mosaic law that punishment must not exceed the crime [or should be equal to it]) and "doing for others what they would want to have done to themselves" (the golden rule [introduced by the Savior]).
Have each class come up with a role-play situation in which it demonstrates the "kind" response. E.g., a new child in the class, a sick child who has to stay home from school, a playground bully, an older person who might be lonely, a mother struggling with small children and groceries, a younger brother who wants to tag along, a child who makes an embarrassing mistake at school, someone you accidentally bump into.
If paper is available, have everyone make a "ruler" to take home as a reminder to "measure' their kind actions. Have them write "Do unto others ... " on the ruler. Follow through if there are actual situations in which the children in your Primary could show a kindness. Finish by singing another "kindness" song.