How would you implement this?
Hardware and firmware combination. Fusible link in the CPU would be the one-way change. Firmware would have to support the service menu option to burn the link forever setting the device to only access the child-friendly internet. The only change in the firmware be the menu to activate the fusible link burning, and later reading the registers where a binary state of "child friendly=0 or 1" exist.
It would require devices so locked down you could never replace or even update the OS on them.
What is child un-friendly about an OS? I can't see any reason why an OS would be labeled not-safe-for-children. I'd be surprised if debian.org or Microsoft.com would be considered child-unfriendly.