This is the wrong implementation.
You require sites hosting adult content to send a header indicating what kind of content it is. Then the device can do what it wants with that information. A parent can then configure their child's device not to display it, without needing anybody to have an ID or expecting every government and lowest bidder to be able to implement the associated security correctly.
It doesn't matter what kind of cryptography you invent. They either won't use it to begin with or will shamelessly and with no accountability violate the invariants taken as hard requirements in your theoretical proof. If you have to show your ID to the lowest bidder, you're pwned, so use the system that doesn't have that.