Sub accounts can’t set parental controls
Go to [Settings] > [Parental Controls].
If you only see [Restrict Use of PS4 Features] and no [Sub Account Management] option, then your child doesn’t have a master account.
Sub accounts don’t have a wallet attached
Go to [Settings] >
[PSN/Account Management] > [Account Information].
If you don’t see a [Wallet] option on this page, your child has a sub account.
This means they won’t be able to create an account with a false date of birth to get a master account.
Go to [Settings] > [Parental Controls] > [Restrict Use of PS4 Features] and enter your PS4 passcode. Select [New User Creation and Guest Login] > [Do Not Allow].
You can take control of the master account to stop your child from using it to get full access to PSN.