First you would create a new build in bootstrap and be sure to only include the grid classes.
Then you would enqueue it with a function like this:
add_action('wp_enqueue_scripts', 'example_enqueue_styles');
function example_enqueue_styles() {
wp_enqueue_style('child-theme-grid', get_stylesheet_directory_uri() .'/bootstrap-grid-file.css', array());
}
This would assume your filename is bootstrap-grid-file.css
and was inside your root child theme folder.
Hope this helps 🙂