We get our subject material primarily from a set of books titled The Golden Treasury of Knowledge (Golden Press, New York.)  and we also include other books we have at home and from the library, as well as the internet.  The Golden Treasury books have information like a beginner's encyclopedia, but they are not in order from A to Z.  So we are always moving on to a completely new subject, and the material covered is not too overwhelming for our children.   What is listed here is only a guideline, and what has worked for us.   Embellish it!  Expand it!  Make it work for your family.