My husband and I (and 2 children) moved to Japan 6 years ago when my second child was almost 20 months. From the time we left my in-laws house until the time we arrived at our new home in Japan, we were traveling for a full 24 hours. Let me tell you, it was a long day! My kids did fantastic though. We packed lots of toys, coloring books and crayons, movies, touch and feel books (my DD fave at the time), snacks and anything else I could think of to keep her entertained for our long trip. Since there was 4 of us we were allowed 4 carry ons and I used all 4. Each child had their own bag of goodies to entertain them, one bag had an extra pair of clothes for each of us and 2 small blankets (airlines don't always have enough to go around) and the other bag had snacks, diapers and other misc things in it. I'm so glad I packed an outfit for each of us, my daughter ended up throwing up when we landed so I ended up having to change both her and I.
Also, my pediatrician told me I could give both my kids Dramamine for the flight. They didn't have a child's version then, maybe they do now, so my doc told me what dosage to give my little ones. Of course, my youngest got a very small amount. Luckily the medicine made her a little drowsy so she slept for awhile on the plane which helped pass the time for her. (Helpful hint in case you decide to use the Dramamine, mix it with a little bit of juice in a medicine syringe, squirt into the inside of her cheek. It'll go down much easier if it tastes like her favorite juice.)
Hope this helps and good luck!