I think the rationale is that the primary way SARSCOV2 is spread is via droplets in the air. The most effective way of doing that is coughing and sneezing, i.e. being symptomatic of a respiratory infection. Children are generally asymptomatic with SC2 and likely remain infected for shorter time frames as SC2 never really gets a strong foothold. These factors make the spreading of SC2 from children likely.