Child account on PS4, how does my child play on their own account and not on the parent account?

I've set up a family account for PS4 and a child account for my child. But how can my child now play on their own account (e.g. Fortnite) if the registration is only possible via the parent account

Which registration? In the game or on the PS4 menu?

In the settings you can switch off the last used user account and protect the others (if I remember correctly) with a password

Independent registration of a child account is not possible on PS4. Or I don't know how to do it. All games and the game itself takes place via the parent account and the child restrictions are pointless. What are we doing wrong?

Well, I don't know what to do now…: /