She has a head cold and we're going to be flying. You might try an antihistamine like Benadryl.
Some antihistamines come in candy-flavored liquids.
They can be poured into soda and the child never knows the difference. Some children might even drink them straight. I'm not quite sure why, but most doctors do not recommend nasal spray for children. For one thing, getting a kid to take nasal spray is sometimes difficult. You've got to get the spray in the back of the throat where the eustachian tubes open up.