On the contrary, for the overwhelming majority of children you're absolutely right, they didn't have serious risk from COVID-19, which makes your question a good one, not a rhetorical one.
Regardless, herd immunity was not a serious possibility at any point (given the high Rº and lack of a vaccine that could prevent transmission), which, considering this was known very early on makes your question, again, a good one not a rhetorical one, despite your intent.
Finally, regardless of herd immunity, at risk individuals would still require vaccination, which makes the herd immunity as a goal, again, irrelevant, which is where I started. It's a nice by-product.
I am not sure about America but in India, I was a child during covid, 7th grade - 8th grade and i didn't have a vaccine but my school students just one grade above us were called in school and they were asked for vaccine.
Though, to be fair, my whole family caught a "virus" during 2nd phase except my father but we didn't go to hospital and just bed rest for 2-3 days. My family really were skeptical of vaccine but personally I don't mind vaccines and would prefer it.
