Travelers must complete a Traveler Health Declaration or "Declaración de Sanidad del Viajero" and present it upon arrival.
Travelers will be tested on arrival (a ‘PCR’ swab and a temperature test), there is a charge of USD 30 for the PCR test.
Travelers must have a certified proof of a negative result of a PCR test taken within 72 hours before travel at an accredited testing center in the country they are traveling from. Children are not exempt. They will still be given a PCR test on arrival in Cuba.
Children under the age of 2 are exempt from presenting a COVID-19 test.
Travelers with proof of recovery from the virus within the last 90 days prior to boarding. Proof of a recent positive viral test and a letter from your healthcare provider or a public health official stating that you were cleared to travel.
More info at the CDC site
Travelers who did not get tested, are advised to stay home and self-quarantine for 10 days after travel.