To the OP. If you're only after 100SCs a W or H return ticket to Singapore will net you 120SCs, that's the beauty of SQ earning full economy instead of discount economy. You could just buy the one Flexi_saver fare and put the rest of the family on non-earning fares on that flight. You'll all end up in the same place anyway. Doesn't matter how you do SIN-KL then either.
Also be wary of anything with a SQ number but operated by SilkAir - it won't earn anything regardless of class. Not so much a problem from Australia, but it can screw you over on the intra-Asia flights.