Kids are not free on most cruise lines.
The one cruise line that does consistently have kids free* is MSC cruises! A few holiday sailings don’t apply.
MSC cruises has kids 11 and under free as the 3rd and 4th passenger on all European sailings (and many Caribbean ones!)
Norwegian, Costa, and Royal Caribbean will have occasional promotions where the 3rd and 4th passenger is free, but this will only be offered on offseason sail dates when kids are in school….sorry!!
Kids are NEVER free on Disney Cruise lines.
*Note “free” passengers still have to pay the government taxes.