Although there have been some reports of myocarditis, or inflammation of the heart muscles, they are very rare compared to the hundreds of millions of vaccines given. It has occurred mostly in adolescent boys and young men, and in almost all cases, it is mild and resolves quickly.
COVID-19 itself is a much more prevalent cause of myocarditis than the vaccines are. It's important to note that studies have shown that the few who developed myocarditis in association with being vaccinated typically have milder cases and recover faster than those who suffer myocarditis as a result of a viral infection.
If you have additional questions, please check with your doctor. He or she knows your and your child's medical history and has your best interest at heart.
SOURCE:
https://jamanetwork.com/journals/jama/fullarticle/2788346 (Journal of the American Medical Association)
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/myocarditis.html (CDC)