The options you are talking about just do not make sense from a financial point of view. As has been stated for the fare difference alone on one direction you could booked the whole family into business class both directions on the longest part of your trip.

I know everyones financial situation is different, but Qantas would be laughing all the way to the bank if you went ahead with what you are proposing. It also would make a mockery of the 'sale' fares you have bought.

I would nearly put this one down to experience and just travel in what you have booked (paying for exit rows maybe) and then in the future don't be blinded by Qantas and you will find some amazing deals out there most likely through Asia mixed class.