Do you mean this comment ?
/* Include the Post-Format-specific template for the content.
* If you want to overload this in a child theme then include a file
* called content-___.php (where ___ is the Post Format name) and that will be used instead.
get_template_part( 'content', get_post_format() );
If so, that is not specific to this theme, it's WP theming in general, and it has nothing to do with making Page become child Page.
To make a Page become a child Page of another Page, see this screenshot below. It's doable within Page editing window, look over to the right at Page Attribute metabox.
To manage its appearance in navigation menu, do it via menu management.