R.K.
We have a 2 1/2 year old daughter and have flown with her several times since birth. I would second everything the person who wrote the first response said - that was excellent feedback. I just want to "second" that it is important not to overpack - be reasonable but keep it to necessities. Resist the urge to pack extra things because you are scared of not having it!
The portable DVD player (while bulky) is a winner with my daughter. She doesn't watch TV very much at home, so it's really fun for her to have it on the plane. The battery lasts 2-3 hours, and I purchased an extra battery so we've never run out on a trip yet. For me, it has been worth the effort of toting it around.
I would also highly "second" on the idea of showing photos of where you are going to your kids, and getting them interested in what they will be seeing and experiencing. Our daughter got to talk to the pilot before our last flight and that was exciting for her. Now when she sees a plane she says airplane and "pilot." She gets it that somebody flies the plane - it's amazing!
There will be a few bumps and/or frustrating moments, but that is okay - try not to panic or spin-out. Stay calm and breathe deeply. Make up games, play I-spy as you are walking around in the terminals (look for colors, glasses, funny shoes, etc. etc.) Also, enlist your kids to help you, too. My daughter holds something 'important' for me, or she pushes her own stroller.
And last, give yourselves plenty and extra time when traveling. Fortunately, when you travel in the US now they have "family" lanes at security -- keep your eyes open for these or ask a security officer and they will point it out. Trying to rush small kids in such a big, new environment can lead to struggles.
I know the idea of traveling with kids isn't fun, but in actuality it can be alot of fun when they are learning and discovering new things. I hope your travels are safe and enjoyable. Good wishes.
R. K.