I think a white listing system could work. Let each country have it's own set of tags (us-7, us-13, ca-violence) which websites can include in headers or meta-tags. Then browsers and (mitm-)proxies could be configured with an accepted list. It would work well for schools, kiosks, kids devices that the parents control, and home routers. Websites would have clear guidelines per country and incentives to comply and include the tags. Encryption and privacy would ensure that the rest of the web operates as it does today; current software wouldn't need to change.