I haven't done it (only with an "easy" aged 9 month old) but I sat next to a couple who had a 2 year old......

Loads of low GI snacks and drinks
DVD player with familiar and long movies (Jungle Book/Nemo etc)
Try and get a late out of Aus flight

Singapore as a stop-over is an ideal option: only 2 hours, and there is a lot of kid friendly stuff (for an airport) to kill time, including, of course, Maccas!

WORSE case scenario, and obviously each to their own, some mild sedative....not sure if I would/could, but I would probably pack some JIC!!

GOOD LUCK!!!!

(We are planning for a UK trip for Christmas 2007.....with a nearly 4 year old, a 2.5 year old and 15 month old......OMG, we must be MAD, so I will be fascinated to see how you go.......)