They do not require a carseat unless they are under 2 years old, HOWEVER I strongly recommend bringing it with. If you child does good with car rides, then especially take it. The first time we took my daughter we did not bring her carseat...the flight was horrible! She wanted to move everywhere and get down and walk around, but she couldn't. When we flew again just a month or so ago, we brought the carseat...no problems. I think because she knows she has no choice but to sit and wait when she's in her carseat, that she didn't fuss very much. She asked me to get out a couple times, but I just told her she has to keep her seatbelt on, and so she was good. We also had lots of activities for her...coloring books & our portable dvd player.
Whatever you choose...Good luck!