How to load a menu item and its children menu items? I have the menu item id. I want to display that menu item with its children. 

Suppose for example I have menu item test and it has two children menu items. So I want to load them and display it as

test
  test1
  test2


I tried to load the menu item with the following code.

$menu_content = current(\Drupal::entityManager()-&gt;getStorage('menu_link_content')-&gt;loadByProperties(array('id' =&gt; 1)));


But I am unable to get the children from the variable. Can someone help me here?
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]);