You are injecting children with substances if you choose to vaccinate them, the question lies whether a vaccine would be effective in children, what side effects might factor in to the cost–benefit analysis (and part of that discussion, how deadly COVD is to children).
Vaccines are generally safe for children. Though I would present the valid concern of myocarditis that can be induced by the vaccine in healthy children, that would not have otherwise been exposed to myocarditis had they gotten a case of asymptomatic or mild Covid.
https://www.medrxiv.org/content/10.1101/2021.12.23.21268276v1.full
In the paper, it's also mentioned that the risk of myocarditis is smaller than that of a Covid infection, which is often used by proponents of mass child vaccination to suggest the outcome of mass vaccination would then result in a lesser net caseload of myocarditis. Of course, as I alluded to above, this fails to discern the gradations of COVID. Not every COVID case is serious, and in fact, the younger and healthier you are, the less chance you have of a severe outcome.
So it would only be valid to compare the cases of mild/asymptomatic Covid myocarditis to the vaccine induced myocarditis instances. For obvious reasons, this hasn't been done because of how difficult it is to track asymptomatic/mild Covid cases in children. The stats just aren't there, countries do not test every child regularly.
I've seen some studies which suggest a second dose of the vaccine in younger males might increase the chance of myocarditis up to 16x.
Seems as if someone at particularly high risk, or in some vulnerable state, would be better off with a vaccine. An otherwise healthy individual doesn't have much incentive to take it, let alone children that are mostly healthy.