Well in the UK you can buy a sim for cash in the supermarket, but to make calls on it you have to activate it, and for that you need a debit card or similar - which makes it linked to an identifiable person. (or a legal entity like a business, with identifiable owners or directors)
Unlike in the films, organizing a truly anonymous phone is quite convoluted, and also illegal, and its not really a problem.
Banks have successfully managed proof of IDs for a long time - and to open a child's account, needs a responsible adult to put their ID to it, usually a parent or legal guardian. I see no reason why getting a digital ID linked to a flesh and blood person who can be arrested if it is mis-used should be any more difficult than the opening of a bank account.
Then like a bank statement, or a phone bill ,the user (or the responsible adult) gets a monthly statement saying what sites have been registered to that ID.much as you see the phone bill on your bank statements.
At this point the responsible adult may look at that statement and query why Jimmy's junior digital ID has been used on some dodgy websites - and Jimmy is in much the same trouble he would have been if caught with a packet of ciggies or a pile of unsuitable illustrated literature ...
What happens next will be as sensible or not as the parents, but there is then at least an official audit trail of who permitted what - unlike now when there are lots of unconnected places holding information.
And that is probably how it will need to work.
Mike.