Don't count on the airline to have what your child needs to eat and drink. Bring along lots of healthy snacks to eat during the flight, including crackers, raisins, dry cereal and fruit snacks. If they're old enough, give them gum to chew on to help with air pressure changes after takeoff and before landing. If they're still drinking from the bottle or nursing, the sucking motion can also help relieve pressure problems.
Flying can dehydrate both children and adults. Make sure your child keeps drinking throughout the flight. Bring a sippy cup or bottle and keep it filled with water or milk.