Coronavirus Disease 2019 (COVID-19) has spread across the world, caused lockdowns, and has had serious economic and social consequences. COVID-19 manifests differently in children than adults, as children usually have a milder course of disease, mild symptoms if any, and lower fatality rates are recorded among children. SARS-CoV-2 transmission also seems to be different between children and adults. Many factors are proposed to explain the milder outcome in children, e.g., a more appropriate immune response (especially active innate response), trained immunity, a lack of immunosenescence, and the reduced prevalence of comorbidities. A better understanding of the differences in susceptibility and outcome in children compared with adults could lead to greater knowledge of risk factors for complicated COVID-19 cases and potential treatment targets. We highlight proposed reasons as to why children are less affected by COVID-19 than adults.