You could always send them with the furniture.
Seriously, they should be kept amused if plugged into the plane entertainment system. Some airlines issue colouring books or sticker books to keep kids amused. We took a portable dvd player (which we forgot to charge up). Otherwise, there is just bribery ... keep a dozen or so new and different toys to placate boredom. Often the trip is so boring that they just fall asleep ... then keep you awake when you've crashed out at your NZ motel. Good luck!