I just flew twice with my 4 year old and 22 month old. It was not very fun the first time, but I learned some lessons. My 4 year old was fine, but my 22 month old son didn't have his own seat, and did NOT want to stay in my lap! He squirmed the entire time. And cried. The next time I flew (a couple weeks later) I did the following things:
I brought TONS of snacks (particularly things that we don't usually give our kids--we brought fruit snacks, Yogos, and Cereal and Milk bars = junkfood), crayons, paper, and a series of new toys that my son hadn't ever seen. Luckily, he loves Hotwheels cars, so I was able to stock up on plenty of them cheaply. Every time he was about to lose it, I opened up another Hotwheels for him. It helped a lot. Also, if you have a portable DVD player, they are very helpful, too. GOOD LUCK!