add_filter("wp_get_nav_menu_items", function ($items, $menu, $args) {

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);

category
   sub-category-1
   sub-category-2
   ...etc

category
   Read all "category"
   sub-category-1
   sub-category-2
   ...etc