If you have any children who will be aged between 2 - 11 years when departing for Australia then generally they get 25 - 33% discount off the adult fare (25% International, 33% Domestic). Infants aged between 0 - 2 years get approximately  25 - 90% discount off the adult fare (25% infant with a seat, 90% lap child).  These discounts vary for different airlines.