As of May 7, 2023, CDC's Advisory Committee on Immunization Practices (ACIP) recommends that all children aged 6 months-5 years receive at least 1 age-appropriate bivalent mRNA COVID-19 vaccine dose. Depending on their COVID-19 vaccination history and history of immunocompromise, these children might also need additional doses* (1-3). Initial vaccine safety findings after primary series vaccination among children aged 6 months-5 years showed that transient local and systemic reactions were common whereas serious adverse events were rare (4). To characterize the safety of a third mRNA COVID-19 vaccine dose among children aged 6 months-5 years, CDC reviewed adverse events and health surveys reported to v-safe, a voluntary smartphone-based U.S. safety surveillance system established by CDC to monitor health after COVID-19 vaccination (https://vsafe.cdc.gov/en/) and the Vaccine Adverse Event Reporting System (VAERS), a U.S. passive vaccine safety surveillance system co-managed by CDC and the Food and Drug Administration (FDA) (https://vaers.hhs.gov/) (5). During June 17, 2022-May 7, 2023, approximately 495,576 children aged 6 months-4 years received a third dose (monovalent or bivalent) of Pfizer-BioNTech vaccine and 63,919 children aged 6 months-5 years received a third dose of Moderna vaccine.† A third mRNA COVID-19 vaccination was recorded for 2,969 children in v-safe; approximately 37.7% had no reported reactions, and among those for whom reactions were reported, most reactions were mild and transient. VAERS received 536 reports after a third dose of mRNA COVID-19 vaccine for children in these age groups; 98.5% of reports were nonserious and most (78.4%) were classified as a vaccination error.§ No new safety concerns were identified. Preliminary safety findings after a third dose of COVID-19 vaccine for children aged 6 months-5 years are similar to those after other doses. Health care providers can counsel parents and guardians of young children that most reactions reported after vaccination with Pfizer-BioNTech or Moderna vaccine were mild and transient and that serious adverse events are rare.