Your keiki may experience mild side effects following their vaccination, such as pain at the injection site, tiredness, muscle or joint pain, or fever. These side effects usually resolve within a day or two.
There have been some reports of myocarditis, or inflammation of the heart muscles, although these cases are extremely rare.
SOURCE: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/myocarditis.html?s_cid=11374:cdc%20covid%20vaccine%20heart%20inflammation:sem.ga:p:RG:GM:gen:PTN:FY21 (CDC)
https://www.hopkinsmedicine.org/health/conditions-and-diseases/coronavirus/covid19-vaccine-what-parents-need-to-know (Johns Hopkins)