Hi:
Wow -- you've got a trip ahead of you! If you're prepared, though, our experience has been that you (and your children) will do great.
We have traveled a lot with our daughter, now almost 5, since she was about a year old. She has her own "briefcase" for short trips (to the doctor's office with me, for example) and a child-size backpack for long trips (a long car ride or a plane ride). I then put into it things she'll like -- a sketchpad, construction paper, child-safe scissors, molding clay, washable markers, a little notepad for scribbling, anything I think she'll enjoy that will occupy her and also make her feel "grown up" for the trip. Into the backpack, I also put a small bottle of water, sanitized handwipes, a ziplock bag of cheerios or goldfish or a snack like this, a special treat (usually a wrapped cookie), and one surprise -- for example, a bobbly-head pen (a decorated pen with googly eyes or something fun on the end). She never knows for sure what the surprise will be and has great fun guessing.
You'll need to be prepared for takeoff and landing -- talk with your pediatrician first, but here's what we've done. When our little girl was young, we'd bring along a bottle or a sippy cup with water or milk in it. As the plane was going down the runway, we'd give it to her and have her drink and swallow as the plane climbed in altitude. The drinking and swallowing motion would keep her ears from hurting, as this motion "pops" the ears (adults do this by simulating a yawn or by chewing gum, but little ones can't). As the plane begins descending at the end of the flight, do the same thing. If the little one won't drink, the special treat you have in the bag for her should come out now -- the chewing motion will also help to clear her ears, plus the food will make her thirsty so that she'll accept the water (by this time, milk will be warm, germs will be building, and it will not be healthy for her to drink).
We also have a portable DVD player but have NEVER had any luck using it to keep our little girl's attention. She likes activity, having something to do, interacting with us, etc.
I've heard of parents giving their children medications like Dimetapp 20 minutes before a flight to make them sleepy, but I just don't like the idea of drugging a child. The exception to this is if the child is stuffy, in which case Dimetapp (for example) would make her more comfortable -- stuffiness and altitude are a very painful combination (I have actually had a ruptured eardrum from it and can't begin to describe how excruciating it was).
Anyway, as I suggested earlier, be sure to talk with your pediatrician for ideas. But our approach has worked every time with our little girl -- to the point that flight attendants and the people seated around us always, without fail, comment on how well-behaved she is, what a great traveler, etc.
Have an awesome trip!