What's the best way to sub-class a PRG based class?

I presume that I can "#INCLUDE < parent.prg >" in each sub-class.prg (so I get the inheritance), but then how do I get the sub-class.prg to re-compile when I edit the parent.prg?

Specifically, if I create a class based on the SESSION object, how can I then sub-class it to get all the inheritance from the main class?