Do I need a child theme for creating a custom post template?

Hi, I created a Custom Post Type using the Custom Post Type UI plugin. I would like to edit the standard template for archives and single posts. I read I need to add a custom template for my custom post type to my WordPress theme.

Since I am using the GP MyPet template, I would like to know, do I need to create a child theme and add a custom template there,
Or do I need to just add it to the core WP files?

Thanks, Mel