Hi - we did this trip in April with a 4 & 2 year old. Who are you flying with? Malaysian?
We were with Emirates and the kids tv was very good. Our 2 year old was very happy watching The Wiggles again and again and again. They also has lots of familiar CBeebies shows. Sticker books are good - funny faces ones and anything else you can find. We also took a few favourite books as we found with the 2 year old, this really helped with the "wind down...it's time to sleep now". Ipad was also helpful - downloaded Peppa Pig episodes were a winner as were the games we had (for the 4 year old)
Snack wise - we took loads of organix bars, rice cakes, bread sticks, small packs of biscuits- I like the cheerios and mini cheddars idea. Emirates kids meals were quite good and the bonus was they came with a snack box which was very handy as one of the children was always asleep during meal time and this helped tide them over between meals.
Re water bottles - just be aware that they have to be empty when you go through security. We had night flights so got a food place to fill them up with milk for us before we got on the plane (Giraffe at Manchester airport, have had Costa do them before as well)
Don't take too much stuff as you will find the children sleep a lot on the flight, particularly if they are night flights. Also, you don't want to be carrying loads of stuff. Our 4 year old took hand luggage but we ended up carrying it a lot!!!
For the flight back (not sure how familiar you are with Oz foods) - packs of tiny teddies, and cruskits were a winner with our kids!
Enjoy the trip & good luck with the flight!