It's not only about blocking access to porn websites but children can access whatsapp groups where child predators are. Why can children access this content in those communication apps? It sounds like providers of these platforms must prevent children accessing those becquse children need to use such platforms to coordinate on their assignments and homeworks. This can be solved by having a "children-type" device account as someone else suggested. The app would know a child is using it and block access. No government id needed.