The latest study I saw suggested that around 30% of children have had covid already.
That is in a year where schools have been closed to most students for a large part of the year and with various strong mitigations such as self-isolation for close contacts, the bubble system, cancelling extra-curricular activities etc and some increased ventilation and mask wearing. And now there is the delta variant which is more transmissible anyway.
Mitigation measures would probably have to cut the transmission rate tenfold to prevent most children getting covid in the long-term and we already know that that is infeasible unless you have a hard lockdown with schools closed.
If you have a five year old starting school, assuming that they won't be vaccinated until they are 12, does anyone really think ventilation, masks and social distancing in schools would prevent them getting covid before they are vaccinated (or that years of that sort of schooling would be worth it if it did). A child's whole school experience could be sitting 2m apart from their classmates with a mask on in a marquee with no mixing between year groups and no extracurricular activities with periodic stints of remote learning, if we implemented the policies which some people seem to want. I think we have lost sight of how damaging even "just" social distancing could be on a long-term basis.
The best that those measures are likely to do is create a timing difference in when a child catches covid, which is not particularly helpful anyway as it is generally better to get it younger.
Of course there may be a change in approach to vaccination at some point or covid rates might fall so much that being infected in school becomes unlikely. In those scenarios there might be a benefit for some children in slowing down transmission as fewer children would then get covid in the meantime. But you could spend years waiting for this sort of change, which might never come, and for many children that will be years of their education with these damaging policies.
These arguments end up being about whether long covid is an issue for children and protecting CEV children but I think that misses the point. Mitigation measures are not going to prevent most children being exposed to covid at some point unless they are much more effective than any measure we have tried so far or we just educate children remotely on a long-term basis.
And I don't think the comparison with flu in the context of children is that far off. Certainly, when I looked up the stats for babies a while ago flu was significantly more dangerous and we don't vaccinate babies against flu or do anything special to stop them catching it.