How do I programmatically change contextual filters?
Args should be concatenated with , or +.

This work:

  $term = Term::load($tid);
  $terms = \Drupal::entityTypeManager()-&gt;getStorage('taxonomy_term')
    -&gt;loadTree($term-&gt;getVocabularyId(), $tid);

  foreach($terms as $child_term) {
    $args[0]  .= ',' . $child_term-&gt;tid;
  }