a) There is evidence of children spreading COVID-19, "children under 15 were about half as likely as adults to be infected, and only half as likely as adults to transmit the virus to others". [1]

b) In order to have children at school, you need teachers at school. In order to have teachers at school, you need public transport. Suddenly, you have a significant amount of people moving around spreading the virus.

[1] https://www.nationalgeographic.com/science/article/we-now-kn...