Hi!
Well, there is no option for that, because if the parent categoriy is not assigned to the child, then it is not considered as part of it. There might be however a solution, by using a custom code.
Try adding this custom code to the functions.php in your theme/child theme directory. Before editing, please make sure to have a full site back-up just in case!
/**
* Scenario: Drop-down mode taxonomy filter, posts in child categories, not included in parent categories
* This snippet will include the child categories, when the parent is selected, and changes the logic accordingly
*
* @param $args
* @return mixed
*/
add_filter('asp_query_args', 'asp_tax_filter_include_child_terms', 10, 1);
function asp_tax_filter_include_child_terms( $args ) {
$taxonomy = 'category'; // Change this to the taxonomy name
// ---- Taxonomy term replacement ----
foreach ($args['post_tax_filter'] as $k=>&$item) {
if ( $item['taxonomy'] != $taxonomy ) continue;
$children = array();
if ( count($item['include']) > 0 ) {
foreach($item['include'] as $tid)
$children = array_merge($children, get_term_children( $tid, $taxonomy ));
}
$item['include'] = array_merge($item['include'], $children);
$item['logic'] = 'or';
}
return $args;
}