[exasperaited]

> I presume the idea is that you don't give little Timmy the password to his own account portal, but I don't know what's to stop him getting his own SIM by himself.

Well — perhaps the toggle is only available if the account has been topped up with a credit card?

One thing that distinguishes you getting a SIM and Little Timmy getting a SIM is that you're over the age of majority and can enter into credit contracts, whereas Little Timmy can only get a debit card.

This fact is actually central to one of Ofcom's recommended age verification techniques, though the adult block on mobile phone networks is much older than these recent measures.