I want to enqueue a .js file to my child theme

Here’s a working example:

add_action( 'wp_enqueue_scripts', 'menu_scripts' );
function menu_scripts() {
wp_enqueue_script( 'responsive-menu', get_bloginfo( 'stylesheet_directory' ) . '/js/responsive-menu.js', array( 'jquery' ), '1.0.0' );
wp_enqueue_script(
    'custom-script',
    get_stylesheet_directory_uri() . '/js/custom_script.js',
    array( 'jquery' )
);
        }

Or like this which apparently loads faster:

function my_scripts_method() {
    wp_enqueue_script(
        'custom-script',
        get_stylesheet_directory_uri() . '/js/custom_script.js',
        array( 'jquery' )
    );
}

add_action( 'wp_enqueue_scripts', 'my_scripts_method' );

Source http://codex.wordpress.org/Function_Reference/wp_enqueue_script

get_template_directory_uri() will only work in a parent theme.