Explain that the captain will light up the seat belt sign on the airplane and no one, not even adults, can get out of their seat for any reason while it's on. Not even to use the facilities. Clue them in on the procedure for boarding. Also, letting them know what to expect from the take off and landing will make them less alarmed when the actual events take place. If your child has been on an airplane before, it still may be a good idea to refresh their memory.
Children are sometimes inclined to forget.
Make sure they understand that the loud engine noises and the bumpy turbulence are normal and does not constitute a reason to panic or anticipate a crash. Now that all that is out of the way, your kids are put to the test. Every time they whine or act up, they have to pay you a penny. Be sure to bring a bag full of toys such as books, drawing materials, travel games, etc. This will sometimes keep them more occupied than what you'd expect.