My girls are now almost 7 and 4. We have traveled with them since they were infants. On our most recent trips we have taken along those magnet activities (my 4 year old loved an easy animal tangram set and they both liked a Noah's Ark one & magnet dolls); colored pencils & paper; stickers; special activity books that we bought for the trip (like one about dogs and one about horses); photo books of the people we were going to visit; a couple of favorite books and a kid magazine with hidden pictures and other games; a deck of cards. We have gone as far as FL to Hawaii with great success.
Another key ingredient to success is talking about what is going to happen ahead of time and being patient yourself. There is a lot of airport wait time. Definitely walk around and look at everything at the airport so that sitting on the plane won't be so hard. Also talk about the airport and airplane rules. You need to keep your feet from bumping the seat in front of you. You can turn on lights and fans, but not push the flight attendant button unless there is an emergency, etc. Children can look at the emergency evacuation instructions, the magazines in the seat pockets, etc.
My girls loved being able to order their own fruit juice or water and getting a special treat on the plane. We also bought special chewies (like fruit chews) for take off and landing so their ears don't hurt. I make sure that we have our own snacks so they don't get too hungry as well.
Hope that helps.
mother of 2 young travelers