How does ECH make it impossible for parents to control their children's access to computers? Sure they can't block sites at the router level, just like your ISP won't be able to block things at the ISP level, but you (the parent) have physical access to the devices in question, and can install client-side software to filter access to the internet.
The only thing this makes impossible is the laziest, and easiest to bypass method of filtering the internet.
The only thing this makes impossible is the laziest, and easiest to bypass method of filtering the internet.