Half-decent BBC article explaining why only 1st doses are being offered to children: https://www.bbc.co.uk/news/health-57888429.amp?s=09

But even then, remember that the difference in risk from covid between unhealthy and healthy is about 10x or so; vax side effects aren't as they're your immune system. So those numbers indicate that for healthy children, it's likely that vaccine induced heart problems alone are worse than covid. And of course, that's not the only side effect.