How do I override part of the main loop in my child theme?

Instead of overriding the main loop, you can always just create a template, similar to the main loop and use that on a separate page (which you then set to display as your home page).

You’d just need to set up a wp_query in that template to call the posts you want and run a loop on that.

That way everything is left in tact and you just have one page in your child theme. Then you can add what you like after the_content();.

That’s how I’d normally go about this sort of thing.

See here: http://codex.wordpress.org/Stepping_Into_Templates