Norwegian Cruise Line was the only major cruise line to not charge for children aged 6 months to 2 years old. However NCL has changed their fares for the young ones and they will be priced the same as other 3rd and 4th persons in a cabin. Newborns that are not yet 6 months old are not allowed to cruise.
The first two passengers in a cabin are priced the same regardless of age on Norwegian. Additional berths in the same stateroom will be at a discounted price, plus port fees and taxes.