apple watch cellular setup on a child iphone with icloud doesn't call

Hello

I have an apple watch 6 LTE that is setup correctly to a child account

The cellular does not connect and cannot call from it whiteout the iphone

apparently because the account is a child account but he has his own iphone with his own number

why apple doesn't allow me to control if I want him to use the cellular function

if he is lost and whiteout the phone only the watch cannot call for help it is absurd

please tell me if I can do anything

Thank you