permalinks of subcategory and posts

use the following code should get you through your problem.

just pass the id of parent category in $term_id variable

<?php
    $term_id =  ;    // id for the parent category
    $taxonomy_name="category";
    $termchildren = get_term_children( $term_id, $taxonomy_name );    
    foreach ( $termchildren as $child ) :
      $term = get_term_by( 'id', $child, $taxonomy_name );
?> 
<h3><a href="https://wordpress.stackexchange.com/questions/191436/<?php echo get_term_link; ?>"><?php echo $term->name; ?></a></h3>
   <ul>
   <?php  
        $args= array('cat'=>$child,
                    'posts_per_page'=>6                                                 
                    );
        $loop= new WP_query($args);                                               
        if( $loop->have_posts()):while($loop->have_posts()):$loop->the_post(); 
   ?>
          <li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>


   <?php 
        endwhile;
        endif;

   ?>

   </ul>
<?php 
    endforeach;
?>