Here are a few links on sixth grade history....
The Constitution and its history use to be taught in schools, but no longer is. The text book they used was "A Familiar Exposition of The Constitution Of The United States. " by Justice Joseph Story. I recommend that all children learn the meaning of the Constitution and its history. It is very interesting. The book is written in old English (word meanings have changed), so one will want to look up important words like welfare, Democracy, and Republic in an older dictionary (free on line version here)
One of my favorites: A game that teaches the Constitution
We are doing Sonlight Core D this year - it's a two part (two years) on U.S. History. So this year we cover up to the Civil War. Next year we cover Civil War through current day.
We have liked a few of the books, so if you don't do Sonlight as a whole, you might like these individually:
The Story of the USA (there are 2 books)
Landmark History of the American People (2 volumes in one - this one is VERY wordy but has so much good information - we often skip several paragraphs at a time, but there are lots of visuals too).
There's a PBS documentary on Netflix about Lewis & Clark - we watched that along with reading a book (we watched it in 20 minute segments each day - took about 10 days to finish). Can also do a lot of mapping activities with L&C.
Pedro's Journal and Walk The World's Rim are good historical fiction for very early American peoples.
Those are just some of our favorites from this year. So I don't have anything to suggest for post-civil war yet... :)
For a secular curriculum I recommend Joy Hakims' books (with the student guide books!). I would also recommend Our Constitution Rocks!.
For a good Catholic US history text, I recommend From Sea to Shining Sea (by the Catholic Textbook Project).
For a good protestant US history text, look into Notgrass' courses (America the Beautiful, I believe it's called).