Find out what the children already know about slavery. Not only is this a good way to judge what new information the children need, you can also find out about misconceptions they might have about slavery. Gently correct their misconceptions as you teach them.
Teach about the history of slavery. Start with world history to place slavery in its largest context. Once children understand that, narrow in on slavery in the United States. When you introduce it, neither serve as an apologist nor a condemner of the practice. This allows children to take a more active moral role, and you can then support their moral and ethical decisions.
Use literature and film to teach about slavery. Fictional accounts reach an audience on an especially empathetic level, allowing them to experience what slaves experienced. You may even want to have them write about their reactions to these accounts.
Read slave narratives to children. "Slavery Time When I Was Chillun" by Belinda Hurmence is a good collection for older children, and all ages might benefit from some passages of the "Narrative of the Life of Frederick Douglass." Discuss the narratives fully to make sure children understand them.
Engage the children with song. Slaves had a rich tradition of song; some were spirituals, some were entertainment and some were coded songs about escaping to the north. Teaching children to sing some of the well-known songs, and teaching them their history, provides insight into the culture of slaves. This can also give you an opportunity to talk about the fact that slaves had a real culture of their own.
Visit sites connected with slavery if possible. A field trip to an old plantation with slave cabins still intact can be an eye-opening experience for children as they can see the vast difference between the "master's" house and the cabins for the slaves. If you can't visit sites, try to locate a nearby museum with artifacts or bring in a picture book for children to examine.
Have children think about the effect that slavery in the history of the United States might still have in contemporary culture, such as racism. This can be a challenging topic for children of any level, so only attempt it if you think the children are ready to approach it openly.
Ask children to find out if slavery still exists, if it's an age appropriate activity. Children can use Internet research to investigate this topic, and you can meet as a group to discuss their findings.