I've flown with children A LOT.
First and foremost--LOTS OF SNACKS!!
Besides that, sippy cup, IPod with movies or cartoons loaded up, books, paper and crayons. A blanket is very important and an airline-approved car seat if child has his/her own seat.
Go to the dollar store and buy bags of tiny dinosaurs, little cars, stickers--pull out a new item whenever he/she gets bored. Also, a package of pipe cleaners might surprise and entertain.
You don't want ear problems and a crying child for hours on end so have child eating and drinking during take-off and landing.