Why cannot the internet/phone provider place them on an "alternative" network with alternative defaults, like kid-friendly DNS and what not? Since the age of the customer is likely to be known at the subscription time.
At home, you would need two connections, one "normal" and one kid-friendly, with another default DNS. You can select which one is the default that would be picked up by the Ethernet cable. Then you just need two wifi network, exactly as you can have "guest access".
Am I being too naive or would this solve like 99% of the problems at very low complexity cost?
escape the browser space, and you will experience the actual internet, a place where no one is supposed to be, a "Darknet" an illegitimate place that is not commercialized, where you have to make your own choices.