Hav you looked at China Southern and other airlines? And also at taking your child out of school during term time if need be rather than being tied to half term. You don't need to make a holiday of this, just a long weekend or something. And to anywhere in Aus that comes out cheapest. Honestly, if its a case of just making the flight, having a few days and coming home then just go for it whenever you can. Don't tie yourself to school hols. They don't mind kids being taken out, its just unauthorised leave is all it'll go on the record as.
I'd not like to wait the extra months for new visas to come through and it still costs. Cost wise I am sure you could get the price of flights down if you go at the cheapest time going between now and when they expire and so on. So there won't be much in it with the 2K visa costs if you waited to get new ones.
I seriously doubt that immigration will extend the visas if they lapse as you would have had plenty of time to validate and you've already entered Aus. I've not heard anyone getting this done. Its usually because of a death in the family or some such that they extend the date of entry by a month or two. They don't seem to give that much longer on the extensions.
I personally would bite the bullet, do some serious online shopping for cheap flights (skyscanner is one site that is good) and book tickets and head over there to validate their visas. Be done with it, sorted and not face lots of hassle and waiting later on.