Lista as subcategorias de uma categoria num select.

form.php

<form action="#" class="form-field" id="ID_DO_SELECT">
  <?php
  $taxonomia = 'ID_DA_SUA_TAXONOMIA';
  $args = array(
    'orderby' => 'name',
    'parent' => 0,
    'taxonomy' => $taxonomia,
    'hide_empty' => 0,
  );
  
  $categories = get_categories( $args );

  foreach ($categories as $category) {
    echo '<div class="form-field-select"><select name="NOME_DO_CAMPO_AQUI" id="NOME_DO_CAMPO_AQUI-'.$category->cat_ID.'"><option>'.$category->cat_name.'</option>';

    $theid = $category->term_id;
    $children = $wpdb->get_results( "SELECT term_id FROM $wpdb->term_taxonomy WHERE parent=$theid" );
    $no_children = count($children);

    // VERIFICA SE A CATEGORIA POSSUI SUBCATEGORIA
    if ($no_children > 0) {

      $args2 = array(
        'orderby' => 'name',
        'parent' => 1,
        'taxonomy' => $taxonomia,
        'hide_empty' => 0,
      );

      $args2["parent"] = $category->term_id;
      $categories2 = get_categories( $args2 );

        foreach ($categories2 as $category2)
          echo '<option value="'.get_category_link( $category2->cat_ID ).'">'.$category2->cat_name.'</option>';
    }

    echo '</select><!--/.NOME_DO_CAMPO_AQUI--></div><!--/.form-field-select-->';
  } ?>
</form>