version-1.php

<?php

add_action( 'wp_enqueue_scripts', 'grappler_scripts' );
function grappler_scripts() {

	wp_enqueue_style( 'grappler-style', get_template_directory_uri() . '/output/style.css' );

	if ( is_child_theme() ) {
		wp_enqueue_style( 'grappler-child-style', get_stylesheet_uri() );
	}
}

version-2.php

<?php
add_action( 'wp_enqueue_scripts', 'grappler_scripts' );
function grappler_scripts() {

	if ( is_child_theme() ) {
		wp_enqueue_style( 'grappler-child-style', get_stylesheet_uri() );
		wp_enqueue_style( 'grappler-style', get_template_directory_uri() . '/output/style.css' );
	} else {
		wp_enqueue_style( 'grappler-style', get_stylesheet_uri() );
	}

}

add_action( 'stylesheet_uri', 'grappler_stylesheet_uri', 10, 2 );
function grappler_stylesheet_uri( $stylesheet_uri, $stylesheet_dir_uri ) {

	if ( ! is_child_theme() ) {
		$stylesheet_uri = $stylesheet_dir_uri . '/output/style.css';
	}

	return $stylesheet_uri;
}

Assuming this code is happening the main [parent] theme:

<?php

add_action( 'wp_enqueue_scripts', 'grappler_scripts' );

function grappler_scripts() {

    if ( is_child_theme() ) {
        wp_enqueue_style( 'grappler-style', get_template_directory_uri() . '/style.css' );
        wp_enqueue_style( 'grappler-child-style', get_stylesheet_uri() );
    } else {
        wp_enqueue_style( 'grappler-style', get_stylesheet_uri() );
    }
}

Or, a simpler version:

<?php

add_action( 'wp_enqueue_scripts', 'grappler_scripts' );

function grappler_scripts() {

    if ( is_child_theme() ) {
        wp_enqueue_style( 'grappler-parent-style', get_template_directory_uri() . '/style.css' );
    }

    wp_enqueue_style( 'grappler-style', get_stylesheet_uri() );
}

Some notes:

• I don't see any reason to use output/style.css at all here. Just stick the code in your main style.css.
• If for some odd reason, you had to use output/style.css, using the filter would be preferred to me.
• Child theme style.css should load second to override parent.