From an early age children begin to move, first they move their arms and legs, roll over and then crawl, soon after they walk and before you know it they are running. Sitting on an airplane (no matter how exciting they think airplanes are!) they want to move.
Connor asks every day if he can go on an airplane, but he's usually also the first one ready to get off the plane so he can run through the terminal building. We don't spend much time sitting in airports because we have learned that we have happier travelers when they have "exercised".
Moving around before boarding is especially important during lay-overs. For most young children the amount of time spent sitting from when you board to when the seat belt sign is first switched off is an eternity. So if you have the opportunity to burn energy before boarding do it!