FWIW I would be ring QF now and getting a rerouted flight to go via East Coast if that is what you want or jump on net and see how much SQ is going to cost.
If I was travelling in economy with 3 kids, (or even travelling in economy alone), I would suck it up and take Jetstar, and not press on the east coast routing. Unless a serious/points status chaser, turning a 5 hour journey into a 13 hour+ journey (with at least 11hrs flying) just seems plain silly. If opposed to flying Jetstar, cancelling and looking at travelling SQ would be the much better option.