Copy WP (child) terms to another taxonomy. Example copies children of term with ID '32' in the taxonomy 'category' (i.e. all my Vocab subcategories) to the custom taxonomy 'sets'. Use in functions.php.

gistfile1.txt

$term_id = 32;
$from_taxonomy = 'category';
$to_taxonomy = 'sets';

$termchildren = get_term_children( $term_id, $from_taxonomy );
 
foreach ( $termchildren as $child ) {
	
    $term = get_term_by( 'id', $child, $from_taxonomy );
    $name = $term->name;
	
	// if the term doesn't already exist in the target category...
	if ( ! term_exists( $name, $to_taxonomy ) ) {

		// ...insert term
		add_action('init', function() use($name,$to_taxonomy) {
			wp_insert_term($name, $to_taxonomy);
		}, 10, 1);
		
	}

}