That's just it. Unvaccinated under 12 don't need tested
The FAQ page spells it out more explicitly
https://www.royalcaribbean.com/faq/questions/will-i-have-to-take-a-test-before-i-cruise
Vaccinated guests do not need to take a test for sailings of 9 nights or fewer. On sailings of 10 nights or longer, fully vaccinated guests age 12 and older will need to bring a negative test result for a COVID-19 test.
Guests age 12 and up who are not vaccinated have three days to take a test before their cruise, regardless the duration of the cruise.
Guests under the age of 12, do not need to take a test prior to cruising.