How do I link a PS4 user account to a child profile?
Solution 1:
First, you will need to create a new user on the PS4 (local account) for the child - this should be fairly straight forward. Once a new user is created, you can then sign into the existing PSN Account.
If you run into issues around "Family Manager" or similar, you may need to first add yourself on to the console by creating a User on the PS4 and signing into your own PSN Account, and then following the above steps to add your child and their existing PSN account.