You need to make the function pluggable, that means, to support child themes, the functions in parent theme should use:  

	if ( !function_exists( 'function_name' )) {
		function function_name() {
			//Stuffs
		}
	}  

You should have seen a PHP Error message if you have [debugging enabled][1].  

**EDIT:** The below code snippet if to answer your other question in comment.  

	function modify_read_more_link() {
		global $post;
		
		return '<a class="more-link" href="' . get_permalink( $post->ID ) . '">Your Read More Link Text</a>';
		
	}
	add_filter( 'the_content_more_link', 'modify_read_more_link' );


  [1]: https://codex.wordpress.org/Debugging_in_WordPress