While kids are less likely to get sick from it (less likely doesn't mean zero cases or even deaths btw), they can sure as hell spread it. There was some research suggesting that they're spreading the virus as much as (or not detectably less than) adults [1]. Yet at the same time, there are also some indications that kids might be less likely to become infected [2]. How that will affect school reopenings is anyone's guess.
It's still a stretch to say that the virus is "harmless" to children.
But that's beside the point. The real point about children is that they can spread the virus just as well, even when they don't get sick - especially since it's also really hard for at least smaller children to keep distance or wear masks.
[1]: https://zoonosen.charite.de/fileadmin/user_upload/microsites... [2]: https://www.nature.com/articles/s41591-020-0962-9