First, that's untrue:

https://data.unicef.org/topic/child-survival/covid-19/

Of course it's not even close to the death rate in adults, especially older ones, thankfully, but it isn't "no risk".

Second, the main reason for school closures was that the kids, even if there's very little risk for them, can transmit the disease to family and friends.

Nonetheless, some countries, like France, made the choice to keep the schools open to maintain some level of schooling for most kids. It wasn't without issues and massive criticism from teachers ( who were risking their lives to keep other people's kids occupied), parents ( because classes were closed when a kid got Covid to prevent further transmission, so you could suddenly have to deal with your kid), etc.