D.T.
We flew cross country dozens of times with our son before moving when he was 2.5. Bring the carseats and use them on the plane. They are used to them and won't mind being buckled in so much. Besides, the adult seatbelts are completely worthless on a toddler and won't restrain him at all. My son slept much better on the plane in his carseat than the once or twice we were insane and flew without his carseat (the only hell trips were the ones without the carseat!).
Bring a small inflatable ball to kick around in the gate area while waiting to board. It's perfect for layovers. When deflated, it takes only a small amount of space in the carryon. Or just leave it in the stroller basket. I would leave a few small 'airport toys', diapers and wipes in the stroller basket. They would stay in the basket when it was folded and gate checked. It's a few less things to carry onto the plane and yet were very convenient inside the airport.
For the 2 year old, bring a portable DVD player with headphones, books with flaps (keeps the fingers busy longer), magna doodle, colorforms (they also stick to the window!), color wonder markers and/or fingerpaint, lacing cards, small matchbox cars (ones with moveable ladders or car doors are better), small containers of snacks -- raisins, goldfish, mini-marshmellows, small pretzels, etc. You can hit the dollar store for a variety of cheap new toys and you wont care so much when parts get lost on the plane. Only pull out one toy at a time on the flight and try to get him to play with it for as long as possible before pulling out the next one. Also don't forget about the onboard 'toys'.... toddlers love those crinkly clear plastic cups they serve water in. Just ask the flight attendent for a few of them and they will happily give them to you. Kids also like to look at the magazines in the seat pockets... and like to rip them up. Let them! Or bring your child's safety scissors to practice cutting on the magazine/catalog.
The best thing to pack is a good attitude! Yes, it's hard. Yes, at least one thing wont' go as planned. Plan on delays and cranky kids. As long as you keep up the "this is a great adventure" attitude, your kids will behave accordingly and you'll be alot less stressed!