Qantas have been known to allow children into the T3 lounge on occasion as "additional guests" but I think it may only apply to those who have status, I'm sorry I'm not 100% sure.
Almost certainly wouldn't work if you aren't flying with Qantas and have no status, anyway.
Centurion probably the best bet as mentioned above.
Hope you have a good flight, genius move putting the kids back in economy
.