Passive reporting systems are pretty much useless for determining rates of adverse events for exactly that reason; they rely on a frontline healthcare worker to decide whether it's related, which often is basically a guess. However, other places have more robust systems where they automatically cross reference these things against vaccination status and when the person was vaccinated, and they're not finding huge increases in adverse events either, so I think the idea that there is this huge undercount of reactions isn't supported by the evidence. They certainly happen, but they're relatively rare.
Obviously the risk/benefit of vaccination changes drastically depending on the age, so the benefit gets less clear as you get younger. Still, there is a pretty clear benefit for all ages. In the last 120 days, about 1 in 2.5K 5-11 year olds has been hospitalized with COVID in Alberta (that's in terms of population, not infections), whereas there hasn't been a single hospitalized case among fully vaccinated in that age group. Assuming a ~30% attack rate in that period, that would suggest that not vaccinating a 5-11 year old is likely risking a 1 in 750 chance of hospitalization or a 1 in 4K chance of ICU admission from COVID in order to protect against a 1 in ~10-20K chance of myocarditis from the vaccine. For an immunologically naive kid that age, I think the math is pretty clear; you're about 10-20x more likely to have a severe case of COVID if you're unvaccinated than you are to have a serious adverse reaction after being vaccinated.
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