Public health authorities regularly perform that kind of risk-benefit analysis. Here's one by the FDA looking at the 5-11 age group: https://www.fda.gov/media/153507/download

My understanding is that this analysis assumed the observed myocarditis risk of the 12-15 age group would also apply here, but IIRC more recent data shows no myocarditis cases at all for the younger group.

There's a theory that links the occurrence of myocarditis to high testosterone levels, but that hasn't been proven yet. Existing data shows that the increased myocarditis risk (more or less) only applies to boys/young males.