Given the answer you mentioned, I assume that you would like to retrieve your items data programmatically.
What you are missing is the
setRoot() method from
MenuTreeParameters (here). Here is how you could start with:
$menuTree = \Drupal::menuTree();
$menuTreeParameters = new MenuTreeParameters();
// This will fetch the tree starting from your Sub-Parent
// Set the nb of levels, starting from root you defined, that you want to fetch (0 = all)
// Load tree based on your parameters
$myMenuTree = $menuTree->load('name_of_your_menu', $menuTreeParameters);
// List of Sub-parent children items
$subtree = $myMenuTree['sub_parent_link_plugin_ID']->subtree;
$myMenuTree will give you data about Sub-Parent as a MenuLinkTreeElement.
$subtree will correspond to its children, each as a MenuLinkTreeElement as well.
In each of these two variables, you will find for each MenuLinkTreeElement a
link entry of type
MenuLinkContent containing all the data about your menu link item.
link, you would be able to use the
MenuLinkContent methods to retrieve the title and url of each of your items.
Hope this helps.