From the P&O website. This is for the cruises out of Malta but the same applies for other cruises.
Guests aged five to 11 years need to be fully vaccinated* with an approved COVID-19 vaccine at least 14 days prior to travel (a booster vaccine is not required) or must provide two negative COVID‑19 test results prior to sailing (see testing requirements below).
In addition to the above lateral flow/antigen test, all unvaccinated guests aged five to 11 years will also be required to have a negative PCR test, organised and paid for by the guest, taken within 72 hours prior to arrival in Malta; the negative PCR certificate must be presented at the airport. This is an entry requirement for Malta.