Domestic flights will typically not charge for an infant traveling as a lap child. An infant is any child under the age of two as of the date of travel. Children in nearly all cases will be charged a full adult fare on domestic flights.
A discount may apply for certain international itineraries for infants and children. Be sure to conduct your search with the number of children and infants included, and be sure date of birth for all passengers provided as infants or children when booking.