When a plane descends and the pressure in the cabin increases, the air pressure in the middle ear must be equalized. If it isn't equalized, the increased cabin air pressure pushes on the eardrum and causes pain. Normally the eustachian tube, a passageway that leads from the middle ear to the nasal passage, will open to equalize pressure. In children, however, the relatively smaller eustachian tube may not function effectively and pain is the result. There are several measures that will help your child more effectively ventilate the middle ear and equalize the air pressure in the ears.
Good hydration is very important.
Airplane air is dry and will thicken nasal mucus, making it harder for the eustachian tube to open. Judicious use of nasal decongestant sprays before takeoff and before descent will also help open the ear and nasal passages. Avoid medications that include antihistamines, such as those for colds and allergies . Antihistamines may worsen the problem by thickening secretions and making your child drowsy. The exception may be if the child is using an antihistamine for an allergy.