Dynamically add sub-categories to any category in the menu

add_filter(“wp_get_nav_menu_items”, function ($items, $menu, $args) {

// don’t add child categories in administration of menus
if (is_admin()) {
return $items;
}

foreach ($items as $index => $i) {

if (“category” !== $i->object) {
continue;
}

$term_children = get_term_children($i->object_id, “category”);

// add child categories

foreach ($term_children as $index2 => $child_id) {

$child = get_term($child_id);

$url = get_term_link($child);

$e = new \stdClass();

$e->title = $child->name;
$e->url = $url;
$e->menu_order = 500 * ($index + 1) + $index2;
$e->post_type = “nav_menu_item”;
$e->post_status = “published”;
$e->post_parent = $i->ID;
$e->menu_item_parent = $i->ID;
$e->type = “custom”;
$e->object = “custom”;
$e->description = “”;
$e->object_id = 0;
$e->db_id = 0;
$e->ID = 0;
$e->classes = array();

$items[] = $e;

}

}

return $items;

}, 10, 3);