Adding PHP to child theme

Hi Guys, I know this topic has been covered, indeed I have tried reading some threads to fathom this, but I reckon I must daft. Having created a child theme on my server and succesfully added a style.css file to my new child theme there is one item of PHP in the parent theme that needs changing, I can change it in the parent theme and my child works, but each time the parent theme gets updated I lose the change in the one line of php code, so I have to go back and find it and change it again.

I would prefer my child theme to be in control of this but have less than zero idea how you constrauct a line of php code such that it is recognised and implemented. Here is the line of code I want to include in my child theme directory ( ‘post-thumbnails’, apply_filters( ‘pagelines_post-thumbnails’, array(‘post’, ‘recipe’) ) );

The ‘recipe’ is the only bit I have to change in the original to allow the posting of a featured image in a plugin, seemingly blocked by the theme I have chosen, but quite like.

I have notepad++ so how, PLEASE, do I create the php file what do I call it and where on my server do I upload it?

Endebted to you people.

Ian