I'm sure you know this, but be sure you get to the airport as early as possible--this past summer, my family of three (I was 7 mos. pg with dd#2 at the time and pretty hard to miss) flew home from the west coast, and as luck would have it, our boarding passes came up with a double X, which meant we had to go through the extra-deluxe super-de-duper security check, with all of our check-in baggage searched, full body searches for all of us, and the hand-carry bags searched. It was horrible--we almost missed the plane, I had to run down the aisle to the gate, only to be searched AGAIN, this time with the guards attempting to separate my 2.5 yo from my leg--needless to say, that didn't work, so they searched around her. We still have no idea why this happened, but I have heard of it happening more and more.
I'm actually considering getting passports for my two girls--we don't plan to leave the U.S. any time soon, but I'd like to have them in the event that we ever do travel again. When I was a kid, my mother got her passport with me listed on it, and with pictures of me in it--the assumption being that we would always travel together. But this was more than 30 years ago. Is it the same now?
Good luck on your trip.