I guess in my mind, teaching a child history or geography or calculus isn't part of a God-focused education.
It certainly can be and should be. Just about every page in my children's science books begin with, "see how God made this..." Similarly with geography. This is less so with elementary mathematics, but in later years children should see the connection between eternal ideas (i.e., mathematics) and their eternal existence in the divine Mind. History, at the most abstract level, is the playing out of ideas in time. History demonstrates the consequences of bad philosophy. For younger children (and also for older children), history should include exemplary stories of heroic virtue.
I see a distinction between the academic and spiritual aspects,
This is a false distinction, and a very serious mistake. All academic studies point ultimately to God.
I think this can be taught by parents even if children attend public schools. I still believe that it is possible for children to learn what I would term "academic" subjects in public school, with their parents providing the spiritual context in which the child lives his or her life and applies their academic learning.