Parental controls available to kid after adding time

I have set time limits for my son in parental controls. Sometimes he would ask me to add time during the weekends. However, after adding time, I found that he could go into parental controls and set time limits from half hour to 5 hours. And sometimes even disable controls altogether.

Is there any way I can add time for him without letting him change all the settings? Is this a glitch?

Thanks.

Jeffrey