That's been borne out by data from several countries, children don't seem to readily spread COVID-19. Adults are the primary spreaders, which is sort of the reverse of the flu.
Compounding this with parents having to bring their children to school, thus generating more contacts than just kids alone.
All in all, there was some data at the root of the policy, but sending kids to school is still a shitty proposition from a safety perspective.
There's no evidence to support the idea that children under 12 are a much risk of spreading COVID-19. According to the CDC which is notoriously conservative, "in-person learning in schools has not been associated with substantial community transmission". According to the NYT [1], NYC school reopening did not drive spread. Here's another [2] look a children 9 and under in Israel showing both low infection rates and low spread in Israeli schools.
We also know that basically no children die of COVID-19. But we do know that for small children being isolated at home is terrible for their mental health and educational futures.
[1] https://www.nytimes.com/2021/03/10/world/nyc-schools-covid-t...
[2]https://jamanetwork.com/journals/jamanetworkopen/fullarticle...