No doubt about it. The concept of slavery in America is a difficult thing to teach children about in the multi-cultural 2010 that some of us live in.
There are a few methods that I’ve come across in books that I own.
In The Pirate Lafitte and the Battle of New Orleans (link goes to actual text) by Robert Tallant (published in 1951), the author uses the “different time” and great men did it” approaches:
Back home in Virginia his father owned many slaves. Now that Mr. Glasscock had moved with his wife and two sons to a plantation in Louisiana, not far from New Orleans, he needed many more slaves to work the fields. Esau new that even George Washington had owned slaves, and so did that other great Virginian, Thomas Jefferson. It was a custom of the time.
In Adventures on Amelia Island: A Pirate, A Princess, and Buried Treasure by Jane R. Wood (2007), the author uses the “different time” tack but adds the dubious “black people owned slaves, too” and “but you got free stuff” arguments:
“And there was a lady here who started out as a slave and then owned some slaves of her own? That’s weird.”
“You have to remember it was a different time. People thought differently then, Planters needed slaves to be able to work their plantations. Slavery was a terrible thing, but the plantation owners provided them with homes, food, and security, and sometimes even gave them their freedom.”
Now in the news, the author of a 2010 vintage Virginia state history book (who is notably not a historian but a self-described “fairly respected writer”) adds the “made up stuff I found on the internet” approach:
“thousands of Southern blacks fought in the Confederate ranks, including two black battalions under the command of Stonewall Jackson.”
In the limited discussions on the topic that we’ve had with our six year old, we’ve gone with the “level with her” and “don’t sugar coat it” approaches with some success. Give it a shot authors.