Kid finds a way into my account using an old PIN

I have set up parental controls. Somehow my sone managed to find the password logged in the Microsoft Familly app and changed the settings at will. I have changed my password in the meantime but he found an easy way around it as he selects use other methods to sign in and then selects PIN, inputs my old PIN and he is back in. How is this possible? I have changed the password, I have changed the PIN, turned on 2FA and reset Windows Hello and he just goes around all this in one go by introducing my old PIN. Is there a fix for this ?