How to load a menu item and its children menu items?
Try these:

// Select the parent menu link with 49 id.
$menu_link_content_id = 49;
// Use the 'menu_link_content' entity storage.
$menu_content_storage = \Drupal::entityTypeManager()-&gt;getStorage('menu_link_content');
/** @var \Drupal\menu_link_content\MenuLinkContentInterface $menu_link_content */
// Fetch an entity with parent menu link.
$menu_link_content = current($menu_content_storage-&gt;loadByProperties(['id' =&gt; $menu_link_content_id]));
// Build a specific value for a parent property.
$parent_prop = "menu_link_content:{$menu_link_content-&gt;uuid()}";
// Load child menu items by 'parent' property.
$menu_link_content_childs = $menu_content_storage-&gt;loadByProperties(['parent' =&gt; $parent_prop]);