Maybe 2 mages! I just remembered - there's no rule that says they can't both play the same class.
[You must be logged in to view images. Log in or Register.] And some classes stack really well: mages, monks, rogues, or wizards x2 = twice the power.
Hey, just thought of another possibility: Iksars. The youngest can play monk, the eldest can play shadowknight, pops can play shaman. SKs only get spells at 9; by that time, he or she may be ready for more complexity.
I'm eager to hear how it goes!