My own belief is that there is no such thing as "college level work," or "high school work," or "junior high" or "elementary" work. Those are arbitrary categories invented in the recent historical past that probably don't fit any children particlarly well.
If it were me, I would teach them as much as they want to learn -- hold nothing back. You don't have to ship them off to college just because they are done with "high school work." They can keep learning more stuff till they're old enough to move away from home, and they'll still be challenged by a good quality college.
