It's actually quite common that vaccinations need to be done multiple times (up to 5 times for DPT) and to have boosters for these when traveling to certain countries.
Are you claiming this as a universal truth? Or your specific experience?
My experience and that of two of my three adult children is that it was like a slight bruise at the injection site for about 24 hours. The other child felt slightly under the weather for a couple of days, similar severity to having a cold. We all had Pfizer.
My impression is that my experience is the more common one, but it is not the one that grabs anyone's attention.
The more serious side effects seem to be vanishingly rare.
so, an inflammatory response.
Covid infection is “potentially” much worse, that’s true, but so is a drive to a vaccination site — the child might die or get seriously injured in a traffic accident. What matters is not the “potential” but rather the actual likelihood, the expected value. Has Covid been dangerous to children? The answer is, rather overwhelmingly, no.
Now that it's becoming vaccinate every six months, the outlook of the cumulative probability of side effects doesn't look that good anymore.
And, going trough an inflammatory process every six months doesn't seem good on its own, even if the vaccine wouldn't have side effect.