I don't want to go off topic, but the reason that most European countries are getting rid of that is because enforcing passports did not really change the outcomes.
Vaccinated (kids or adults) can get and transmit covid, so that won't prevent contamination/spread in the ship. So What I think they will do is to get rid vaccine mandates all together and test both kids and adults before boarding. They might still be negative when boarding when they have the virus (incubation period), so at the end of the day, but that might reduce the likelihood of transmission but does not eliminate).
Covid won't go away, unfortunately. you can't contain a highly contagious respiratory virus.