Can menuTree load a menu's items including children
Within the typical route context ($menu_tree-&gt;getCurrentRouteMenuTreeParameters($menu_name)), subtree elements are loaded only, if the menu item is flagged as "Show as expanded" within the menu item settings.

To load the entire tree, you have to pass in a plain MenuTreeParameters object without the current route context:

use Drupal\Core\Menu\MenuTreeParameters;

$parameters = new MenuTreeParameters();

// Optionally limit to enabled items.
$parameters-&gt;onlyEnabledLinks();

// Optionally set active trail.
$menu_active_trail = \Drupal::service('menu.active_trail')-&gt;getActiveTrailIds($menu_name);
$parameters-&gt;setActiveTrail($menu_active_trail);

// Load the tree.
$tree = \Drupal::menuTree()-&gt;load($menu_name, $parameters);