It is certainly NOT worth a $1800pp upgrade to premium economy - no matter how you look at it.

The other option you have is to purchase exit rows for the three eligible passengers (this would be $160 for the through flight), with one passenger sitting in the row behind (13 years - not eligible for exit row).

Alternatively, you could use a fraction of theat money and break your journey in SIN with a fantastic hotel for a fraction of the cost, complete with transfers and all meals. Even something like the Raffles hotel and you will still be way out on top.

Or your money would pay for lounge access for the whole family (a couple of qantas club memberships)

The only advantge of premium economy is extra width and leg room. The meal service is slightly upgraded, but the champage is gone (replaced by sparkling wine), and the noise cancelling headseats... well... you could buy and keep many pairs for the $7500.

For almost the same money as your original fares plus the upgrade you could have taken advantage of a mixed fare itinerary - flying economy on Qantas from australia to Asia and then connecting on business class from Asia to Europe (swiss). That would have been confirmed business class for 4! (I know its too late now though).