One answer is the Online Library of Liberty. This organization has put nearly every fundamentally important work on their website available free and each work is downloadable in a variety of electronic formats, including pdf scans of the original texts.
I had no idea it was possible to get my hands on some of these books, let alone have them in my house to read.
You could let your middle and older children loose in this site to explore and read, and the history education they would come away with would be phenomenal. Far beyond what most of us came away with after our schooling years.
Let your children touch the past and read the words of those that came before us. Understanding those fundamental thoughts and ideas will lead only to an understanding of the gifts of liberty we enjoy today.