Recollect when you traveled in an aircraft for the first time, the feeling you had. There is a lot of vibration when the aircraft taxis on the runway at a very high speed and then takes off. At this time there is change in air pressure inside the cabin which directly has effect on the ears. You feel compression and ear popping sensation and may have to apply pressure from inside to be normal. As you travel subsequently you are less scary and do not feel that way.
When flying, you may have noticed some babies crying when the plane takes off and climb or when it starts to descend. In some cases this may be due to the fact that the kid is frightened and starts to cry loud as a resentment or fear. However, in majority of cases the child cries due to discomfort felt in the ears due to increased cabin pressure. Small children are unable to understand as to what has happened and why it has happened. They are unable to adjust to the change of cabin pressure due to the changed altitudes. They may need some help at this time.
The child may feel pain in the ears if they are unable to clear the ears during takeoff and landing of the aircraft. To overcome this problem in case of nursing children the mother can help the child by nursing him or her at the time of takeoff or landing. A bottle of milk works at this time. The child sucks milk from the bottle, and then swallows it resulting in clearing of ears. Some children may vomit the milk and they may be given some decaffeinated fluid. In case your child is being breast fed this will help the most. Likewise in case of slightly grown up child, if the child is given a snack to chew and eat it will also help. Chewing gum should be avoided at the time of takeoff or landing though it can be of help during the remaining part of the flight. If the child can yawn intentionally let him yawn to clear the ears.
This sensation caused in the ears of the children is a temporary phenomenon and lasts for a very short time, and gets cleared on its own in minutes after the flight. If due to any reason the child is unable to clear the ears and complains of uneasiness even after the flight, a visit to the paediatrician may be necessary.
If your child has ear infection or nasal congestion you must get the blockage cleared by some medicine as may be prescribed by your doctor. He may prescribe some decongestant. This will help minimise the effect of changing cabin pressure on the delicate ears of the kid. It will be better if you do not go in for self medication, use medicine prescribed by paediatrician as he has more experience than you.