In the case of influenza or respiratory syncytial virus, children are often more severe than adults. However, in the case of COVID-19, children infected with SARS-CoV-2 have a milder clinical course with significantly less morbidity and mortality than adults.
Several hypotheses have been proposed to explain why children are more protected than adults, for example, expression of angiotensin-converting enzyme 2 (ACE2) is lower in children, resulting in lower viral loads, presence of antibodies to common cold coronaviruses （229E, NL63, HKU1）that might provide partial protection is stronger in children, and a more robust innate response in children in early course of infection that mitigates against a vigorous adaptive response.
A group from Albert Einstein College of Medicine, etc. has discovered that nasopharyngeal mucosal immune response is stronger in children. It is speculated that the immune response is strengthened in nasopharyngeal mucosa because children have more frequent respiratory infections than adults. Measured cytokines in nasopharyngeal swab fluid (IFN-γ, IFN-α2, IL-1β, IL-8) was significantly upregulated in children than adults. So, the robust mucosal immune response lowers viral loads and gives protection against advancing in severity.
https://insight.jci.org/articles/view/148694
