This was true but it has changed. The new protocols do not require kids under 5 to test at all irrespective of vaccine status.
Guests 5 years of age and older who are not fully vaccinated must bring a negative test result for a COVID-19 antigen or PCR test taken within the 3 days before boarding day, regardless of the duration of their cruise.
Fully vaccinated guests 5 years of age and older must bring a negative test result for a COVID-19 antigen or PCR test taken within the 3 days before boarding day on sailings that are 10 nights or longer only. Testing is not required for fully vaccinated guests on shorter itineraries.
Guests younger than 5 do not have any testing requirements.