Wild theory: In the majority of European countries some form of partial/selective lockdown strategy was adopted. By in large kindergartens and primary schools

Wild theory: In the majority of European countries some form of partial/selective lockdown strategy was adopted. By in large kindergartens and primary schools have remained open. So far the official communication was that children/toddlers could not develop severe symptoms (We now know it's not quite true) could not be a meaningful transmission source (low viral load?). What if the new strain evolved from the selective pressure of the opportunity of kids being a vector between locked-down households?