Whether children should be vaccinated against COVID-19 (or other infectious diseases such as influenza) and whether some degree of coercion should be exercised by the state to ensure high uptake depends, among other things, on the safety and efficacy of the vaccine. For COVID-19 these are currently unknown for children, with unanswered questions also on children’s role in transmission of the virus, the extent to which the vaccine will reduce transmission, and the expected benefit (if any) to the child. Ultimately, deciding whether to recommend that children receive a novel vaccine for a disease which is not a major threat to them, or to mandate the vaccine, requires precise information on risks, including disease severity and vaccine safety and effectiveness, and comparative evaluation of the alternatives, and of the levels of coercion associated with each. But the decision also requires balancing self-interest with duty to others, and liberty with utility. Separate to ensuring vaccine supply and access, we outline three requirements for mandatory vaccination from an ethical perspective: (1) whether the disease is a grave threat to the health of children and to public health; (2) positive comparative expected utility of mandatory vaccination and (3) proportionate coercion. We also suggest that the case for mandatory vaccine in children may be strong in the case of influenza vaccination during the COVID-19 pandemic.