Hi A.!
We lived in Japan for 25 months, so made lots of these trips back and forth with our kiddos. My daughter was the hard one on the flight. She was 4 when we left. Here are some ideas:
Don't take the carseat onto the plane. It's just too long to sit in one place. Our daughter thought she was really big not to have a carseat, but get the same belt we did. We loosened the belt a little so she could sleep (take jammies, diapers, pullups or whatever!) We told her before we left we'd get on the plane, play, eat and when it came time for night, we'd put her pjs on and get her animals out and tell her to sleep. Some flights it worked; some it did not. We took her favorite toys (all in separate QT ziploc bags for easy clean up and easy finding in a backpack). One idea for your sanity is that only one "toy" out at a time. That way you can keep track of what is out and what isn't and hopefully not lose things. The portable DVD player is an excellent idea because it'll keep her busy for a long period of time. Do you have a Leapster or something like that? Those are also great. Another thing we did was to get a small rolled-suitcase (you know those little ones like for an overnight?) for her to take through the airport. That can hold her pjs, blankies, special stuffed animals (that for my daughter only came out at 'sleep' time), etc. You can decide if she carries the electronic equipment!
Tell her what to expect before you get onto the airplane and how you know she'll be a big girl and help mommy. Tell her when it's time to go to sleep, you'll put on her pjs and that's her cue to sleep. We talked to both kids until we thought there wasn't anything else to tell them, but when we got to the airport, we'd retell them (...and now we go through security...and now you'll roll your suitcase to the gate...and now we get onto the airplane...and now...) as we went and they seemed to understand better.
No matter what you do, it's still a long flight! Some of our friends who lived there with us would give their active kids Benedryl so they would sleep on the airplane. Our doctor told us that was definitely an option...if that doesn't hype the kid. We didn't medicate the kids and our jet lag period seemed shorter...but who knows?! You certainly wouldn't be faulted if you tried it!
I hope you are able to sleep some on this trip. The Japanese are very helpful once you land, but it's always easier if you're at least a little rested.
Good luck!