Cannot limit play time to my kid /Administrator account

[Pbetancourt]

Hello, I'm trying to limit the amount of time my kid plays while I'm not at home. The problem is that when I bought the play 3 years ago, I just created one account, with my email in order to let my kid play. So that account has the privilige of an administrator. That means that if I create now a sub account in order to use the parental control and establish a time limit, my son will loose all his buys and achievments (I supposse). Is there any way to transfer the administrator privilige to another user? Or any workaround you can suggest to establish a limit, avoiding any personal projection and opinions such as "let him play" or "prove him who rules at home". Much appreciated