I feel your pain. We have the same dynamic -although oldest is seven.
Everything has to get bigger - house/car/fridge.
We booked two rooms with interconnecting door for a few days at the Paradise Resort on the Gold Coast.
I think, from memory it was among the cheaper options.
I found it pretty horrendous, but I may be in the minority as others I've spoken to about have raved about it.
If its for a week or so have you looked at renting a self contained apartment on the Gold Coast - Palm beach, etc?
Wotif/Last minute, stayz, etc can give you some idea on costing, etc, if you haven't looked at them, yet..
Best of luck - sounds like a fun trip!