GED could work for the older child, as it seems as if the transfer of credits will be an issue. I was in a similar situation and just decided to take what the high school offered, though this was likely not the better option. For the younger one, could he take AP Calc at the high school and then take the upper division ones at the university? Also, he could try to take the AP classes for the high school and then move to the university for upper division classes. This worked pretty well for me, even though not all of the classes transfered to my high school...