CAR SEAT. Bring the car seat. We got a great car seat carriers from Right Start that ours slipped into and we were able to carry them (back in the days when we travelled with THREE car seats and TWO umbrella strollers -- single and double) on our backs or like duffle bags. They aren't that hard to carry and are so valuable on the plane....really, do it. Our kids are the types that get hyper and cranky on benadryl so drugging them has never been an option for us. If you want to choose that route, do a couple test runs first to make sure you child reacts the way you want.
In terms of kicking the seats, what has always worked for us was to remove the child's shoes once we board. That makes the kicking less awful if it happens. In addition, we always introduce ourselves to the people directly in front of our kids, tell them that we have warned the guys about inadvertantly kicking the seats and that if it does happen, to please let us know asap so we can try to find a solution to it. I have found that such a preemptive strike really goes a long way towards fostering harmony with our fellow passengers.
Leapsters, sticker books, playdough, pipe cleaners, coloring books, portable dvd players, snacks, new toys (or dvds, games), books, matchbox cars, bring along thomas (not takealong) have all helped us keep the kids occupied.
Finally, we are firmly in the "be the last to board" camp. There is nothing gained by being on first with little kids. My DH usually goes on ahead and installs car seats and whatever else needs to be done and then I let the guys run around and burn off energy until they are about to close the doors and then we hop on. Minimizing the time on the plane is our goal.
Have a fun trip. If you plan for it, it really won't be awful. Just bring that car seat!
Taitai