<section class="" style="">
  <div class="container">
    <div class="tabwrapper">      
      <style>
        /* Style the tab */
        .tab {
          overflow: hidden;
          border: 1px solid #ccc;
          background-color: #f1f1f1;
         }
        /* Style the buttons that are used to open the tab content */
        .tab button {
          background-color: inherit;
          float: left;
          border: none;
          outline: none;
          cursor: pointer;
          padding: 14px 16px;
          transition: 0.3s;
         }
        /* Change background color of buttons on hover */
        .tab button:hover {
          background-color: #ddd;
         }
        /* Create an active/current tablink class */
        .tab button.active {
          background-color: #ccc;
         }
        /* Style the tab content */
        .tabcontent {
          display: none;
          padding: 6px 12px;
          border: 1px solid #ccc;
          border-top: none;
         }
      </style>
      <script>
        function openTab(evt, tabName) {
          // Declare all variables
          var i, tabcontent, tablinks;
          // Get all elements with class="tabcontent" and hide them
          tabcontent = document.getElementsByClassName("tabcontent");
          for (i = 0; i < tabcontent.length; i++) {
          tabcontent[i].style.display = "none";
           }
          // Get all elements with class="tablinks" and remove the class "active"
          tablinks = document.getElementsByClassName("tablinks");
          for (i = 0; i < tablinks.length; i++) {
          tablinks[i].className = tablinks[i].className.replace(" active", "");
           }
          // Show the current tab, and add an "active" class to the button that opened the tab
          document.getElementById(tabName).style.display = "block";
          evt.currentTarget.className += " active";
         }
      </script>
      <div class="tab">
        <button class="tablinks active" onclick="openTab(event, 'tab1')"><?php echo get_cat_name(8);?></button>
      <?php
        $parent_id = 8;
        # получаем дочерние рубрики
        $sub_cats = get_categories( array(
          'parent' => $parent_id, # or 'child_of'
          'hide_empty' => 0
        ) ); ?>
        <?php $counter=2 ?>
        <?php if( $sub_cats ){ foreach( $sub_cats as $cat ){ ?>
          <button class="tablinks" onclick="openTab(event, 'tab<?php echo $counter ?>')"><?php echo $cat->name ?></button>
        <?php $counter++ ?>   
        <?php
         }
          wp_reset_postdata(); // сбрасываем глобальную переменную пост
         }
      ?>
      </div>
      <!-- Статичный таб -->
      <div id="tab1" class="tabcontent" style="display: block;">
        <?php
          $posts = get_posts( array(
            'numberposts' => 10,
            'category'    => 8,
            'orderby'     => 'date',
            'order'       => 'DESC',
            'include'     => array(),
            'exclude'     => array(),
            'meta_key'    => '',
            'meta_value'  =>'',
            'post_type'   => 'post',
          ) );
          foreach( $posts as $post ):?>
            <h3 style="margin-bottom: 36px;"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
          <?php endforeach; ?>
        <?php  
          wp_reset_postdata(); // сброс
        ?>
      </div>
        <?php
          $parent_id = 8;
          # получаем дочерние рубрики
          $sub_cats = get_categories( array(
            'parent' => $parent_id, # or 'child_of'
            'hide_empty' => 0
          ) );
        ?>
        <?php $count=2 ?>             
        <?php
          if( $sub_cats ){
          foreach( $sub_cats as $cat ){
        ?>       
        <?php
          # получаем записи из рубрики
          $myposts = get_posts( array(
          'numberposts' => 10,
          'category'    => $cat->cat_ID,
          'orderby'     => 'post_date',
          'order'       => 'DESC',
        ) ); ?>
      <div id="tab<?php echo $count ?>" class="tabcontent">
        <?php $count++ ?>  
        <?php
          # выводим записи
          global $post;
          foreach($myposts as $post){
          setup_postdata($post);
        ?>
          <h3 style="margin-bottom: 36px;"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
        <?php
         }
        ?>  
      </div>
        <?php
         }
          wp_reset_postdata(); // сбрасываем глобальную переменную пост
         }
        ?>
    </div>
  </div>
</section>