Delete thumbnails when deleting the PDF file

Hello. I would like to suggest that pdf thumbnails are deleted when a pdf is deleted.
I’ve added the following code in my custom theme to this end:

add_action( 'admin_init', 'codex_init' );
function codex_init() {
    if ( current_user_can( 'delete_posts' ) )
        add_action( 'delete_post', 'delete_child_attachments', 10 );
}

function delete_child_attachments( $pid ) {
	if( 'attachment' == get_post_type($pid) ){
		global $wpdb;
		//get any child attachments - pdf thumbnails are saved as an attachment of the pdf file
		$child_ids = $wpdb->get_results("
		  SELECT ID
		  FROM $wpdb->posts
		  WHERE
			post_parent = $pid
			AND
			post_type = 'attachment'
		");
		if( !empty( $child_ids ) ){
			foreach( $child_ids as $child_id ){
				wp_delete_attachment( $child_id->ID , true );
			}
		}
	}
	return true;
}

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