short code edit - subcategory label

I have been using this code, now trying ot edit it to have a label that would disappear if there were no subcategories.

Any help for a php newbie?
What I added echo '<p>Available Subcategories</p>';
shows it, but no functionality of course.

add_shortcode( 'list_subcats', function() {
    ob_start();
    
	echo '<p>Available Subcategories</p>';
    $current_cat = get_queried_object();
    $term_id = $current_cat->term_id;
    $taxonomy_name = 'category';
    $term_children = get_term_children( $term_id, $taxonomy_name );
	
    echo '<ul class="list-subcats">';

    foreach ( $term_children as $child ) {
	$term = get_term_by( 'id', $child, $taxonomy_name );
	echo '<li><a href="' . get_term_link( $child, $taxonomy_name ) . '">' . $term->name . '</a></li>';
    }

    echo '</ul>';

    return ob_get_clean();
} );