Air travel is sometimes associated with rapid changes in air pressure. To maintain comfort, the Eustachian tube must open frequently and wide enough to equalize the changes in pressure. This is especially true when the airplane is landing, going from low atmospheric pressure down closer to earth where the air pressure is higher.
Actually, any situation in which rapid altitude or pressure changes occur creates the problem. You may have experienced it when riding in elevators or when diving to the bottom of a swimming pool. Deep sea divers are taught how to equalize their ear pressures; so are pilots. Your child canlearn the tricks too.