Transfer Fortnite account from parent PS to KinderPS account, how does it work?

My child linked their Fortnite account to my PS4 account. But I want it to play on its child PS account so that the parental controls work. The transfer of the Fortnite account from parents to KinderPS account does not work. What are we doing wrong?

I would ban Fortnite! This is such junk! If your child is so young that they need parental controls, they shouldn't play the game at all! Is from 12

My child is 13.

Why do you need parental controls at 13? I know someone who is 13 and precisely because of such restrictions they are too incapable of thinking!