Thanks for writing in! We do not recommend to rename child theme folder as it will cause issues. For X theme,
x-child is the correct child theme directory and for Pro theme, it's
If you want to re-install a new child theme, you can rename your exiting child theme to any name, then you can setup your fresh copy of your child theme and then you can move your old child theme templates to the new one. We also have a child theme setup guide here (https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57).
Hope that helps.