We have not seen myocarditis following vaccines in 5–11-year-old children, but data are only available for approximately 3,100 children. Myocarditis following COVID-19 vaccines is most common in people assigned male at birth, ages 16 to 25, and has been seen very rarely in children ages 12 to 15, including boys.  The CDC reports that the risk for myocarditis in teenagers (12-15 year olds) assigned male at birth after the 2nd dose of vaccine is estimated to be 20-60 cases per 1 million doses (approximately .002% to .006% of vaccinated children).
The risk of heart problems including myocarditis in children admitted to the hospital with COVID-19 is much higher (about 2-10% of admitted children) and generally much more serious than the risk of myocarditis after the mRNA COVID-19 vaccine.