Studies show that COVID-19 vaccines are safe and effective. Like adults, children may have some side effects after COVID-19 vaccination. These side effects may affect their ability to do daily activities, but they should go away in a few days. Children 12 years and older are now eligible to get vaccinated against COVID-19. COVID-19 vaccines have been used under the most intensive safety monitoring in U.S. history, including studies in children 12 years and older. Your child cannot get COVID-19 from any COVID-19 vaccine.
http://www.cdc.gov/coronavirus/2019-ncov/vaccines/faq.html?s_cid=10492:what%20is%20in%20the%20pfizer%20vaccine:sem.ga:p:RG:GM:gen:PTN:FY21