Thank you for showing me that article. I think it's because the antibody responses were very efficient that it limits virus replication. As the paper stated:

"However, the children had similar cellular and SARS-CoV-2 specific antibody-mediated immune responses to their parents, suggesting that the children were infected with SARS-CoV-2 but, unlike the adults, mounted an immune response that was highly effective in restricting virus replication."

This makes sense given that increasing age comes with increasing risk of more severe Covid-19.