For domestic flights there is usually no difference between an adult fare and a child fare, however certain fees or taxes might be lower. On international flights, child airfares may be charged at 50% to 80% of the full fare depending on the airline and route.
Can a 2 year old fly free on Southwest?
One child over 14 days old and under two (2) years of age not occupying a seat may be carried free of charge when traveling with someone 12 years of age or older. If you plan to travel with an infant as a Lap Child, you must pay the taxes and fees that apply to the international portion of the infant's itinerary.