Need to add version number to child theme to avoid cache issue

I have added the following code to my child theme functions.php file to append version number to child theme style.css. But the version number is not getting appended to the child theme style.css

$child_theme_url = get_stylesheet_directory_uri();
$theme_num = wp_get_theme()->get('Version');
wp_register_style( 'avia-style' ,  $child_theme_url."/style.css", array(), $theme_num, 'all' );

The output for the above should be

<link rel='stylesheet' id='avia-style-css'  href='/wp-content/themes/mychildtheme/style.css?v=1.0.1' type='text/css' media='all' />

but it only show as

<link rel='stylesheet' id='avia-style-css'  href='/wp-content/themes/mychildtheme/style.css' type='text/css' media='all' />

What am I doing wrong?

Thanks