Automatically expand all top-level terms

<?php

// Add to your (child) theme's functions.php file

add_action( 'wp_head', function() {
?>
<script>
(function($) {
  $(document).on('facetwp-loaded', function() {
    $('.facetwp-facet-product_categories > .facetwp-checkbox .facetwp-expand').each(function() {
      if ('[+]' == $(this).text()) {
        $(this).click();
      }
    });
  });
})(jQuery);
</script>
<?php
}, 100 );