Starting in 4th grade, we have, on and off, used free university courses:
https://www.coursera.org/
Until about 9th grade, I'd participate side-by-side with our child. We had a great time with discussions, finding additional resources at our library, choosing supplemental DVDs and field trips, making up projects (e.g. interviewing some of the older folks at church who were WWII vets), etc. I geared these classes to be as much or as little work as we were prepared to do given the rest of our lives and given the interest level of the child.
The material was never over the boys' heads, and it gave them a lot of confidence that they could take charge of their education and challenge themselves.
peace,
tessiemae