Stickers have been a life saver for our family! My niece will stay busy with stickers for long periods of time on flights.
The portable DVD player with their favorite shows also helps a lot! Just be sure to bring small head phones for them. Not everyone is a fan of the Wiggles unfortunately.
I also like the idea of small individually wrapped presents. My mom used to do that on car rides. We got to open a new one every hour. On a flight, you could do every half an hour.

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