How do I say: if parent = _____ then _____

you may need to explain what you’re actually trying to do as that’s exceptionally vague…

I am trying to have it so if a page is a child of a certain page to display something there.

something along the lines of:
if (parent=Testimonials)
get_template_part( ‘loop’, ‘testimonials’ );
}

Assuming the page id of Testimonials is 123, then use

if ( 123 == $post->post_parent ) ...

See https://codex.www.ads-software.com/Function_Reference/get_post for other properties of the current post that you can reference in the same way.

Note 1: You can use a plugin to display the page id – e.g. Reveal IDs or simply hover over the edit link in the All Pages screen and the ID will show up in the edit URL.

Note 2: If you want to use the page slug (e.g. ‘testimonials’) instead of the ID, see the example https://codex.www.ads-software.com/get_posts#Get_a_post_by_its_slug but change “‘post_type’ => ‘post’,” to “‘post_type’ => ‘page’,”.