Rooflyer what would you suggest is the easiest way to find the answer to a & b? I have searched the Qantas site again (sucker for punishment), with no luck (perhaps it's the user!)
Make a dummy booking on the Qantas site; go as far as just
before you have to give credit card or agree to the points redemption !!. You should see what the points charge is, and the cash surcharges. I don't know where they draw the boundaries for children; if yours are all 15-ish they'll all be treated as adults for fares and redemptions; I'm sure it says what the age brackets are (Infant, children, adults). If they are <10 it might save money by making that the award seat OR it might be cheaper if that was one of the paid seats.
When making the dummy bookings, make sure you use both Qantas and Emirates flights as options. Fares can be different, but SC and points earns can be too.
When you make the dummy bookings for the paid fares, click on where it gives info for the type of fare; the very cheapest will probably not earn SCs and points. If its a dirt cheap fare, it might be worth the trade off, but the fare will be inflexible.
Since Qantas charges these cash surcharges on 'Award' seats, it has been known for a paid, dirt cheap sale economy fare to be LESS than the surcharges on a points redemption seat! Crazy.
One thing about this game is that there are rarely quick answers - you need to put some time in, as you have started to do. BUT AFFers love to help too. Go to the Qantas part of this forum and post questions there - but show that you've done a bit of research. You won't always get the same opinion, but 'how to' type questions can usually be answered.
Oh, and maybe go over to the 'introductions' forum and say hi there.