Perhaps you could show a photo of a friend who is a different race than you. Don't say this is a person from a different racial group, just talk about your friend and let children make the connection. Maybe you could visit a hospital maternity ward in a diverse part of town, or store that celebrates a particular heritage. Police and fire departments also make good field trips and give children an opportunity to see many types of people in action. Try to find places where people are not in traditional roles.
Foods can be a great way to introduce diversity.
Invite parents or restaurant owners to bring in dim sum, matzo balls, collard greens, tacos, or other surprising food. Serve sandwiches from different parts of the world. The Foods You Eat, a curriculum by Many Hands Media, and Yoko, a storybook by Rosemary Wells about a little cat who provokes an uproar at school by bringing sushi for lunch. Sometimes teachers will intercede in children's disputes before the children have had a chance to talk it out. Help your children trust their own abilities to solve their problems.