It’s not as if kids coming out of lockdown are suddenly exposed to a disproportionately higher number / load of virii than if they were exposed at a younger age.
What do you mean, exactly? Yes, the situation seems to be that kids coming out of lockdown are suddenly exposed to a disproportionately higher number of new virii in a short time than if they had been gradually exposed to them over the previous two years.
I mean that the viral loads they'd be exposed to in year 3 (post lockdown) would be the same as the loads they would have been exposed to in year 1 if there had been no lockdown.
A person's immune system is different in year 1 than in year 3 (and in basically all other variations of N and N+2).