Here are the instructions that the player should follow on their own app to claim their account when the parents have created it already:
Press Join team on the start page & enter the team code
Select > I'm a player
Add a birth date
Enter their own email > wait for the code & enter it
Select their name from the list of players (this is the account that is linked to their parents) > request access to account.
Then the parent should receive a request for connection in your You tab, which they can approve.
Both the player and the connected parent(s) can now manage this account.