It is about transmission, not immediate danger to the kid.
Based on European and Israeli schools data, 15 year olds are old enough to transmit the disease to each other, but 9 year olds are not. (age numbers came from the WSJ article on Israeli school re-closures.
I don’t agree with the theory that it is safe to allow the disease to spread and rely on a firewall around the vulnerable. The firewall inevitably leaks. We’re better off controlling the spread.
To answer kicker’s question, I have kids in both age ranges. The older two are having to make do with bike rides and minecraft, just like the rest of the kids in their classes. They’d love their quarantine to be over, too. The numbers just don’t support it.