Limit the menu child levels under “admin > appearance > menus”

Just use remove_submenu_page( $menu_slug, $submenu_slug );.

• $menu_slug = Parent page
• $submenu_slug = Submenu page

Example usage:

// Remove the submenu page for widgets
add_action( 'admin_menu', 'adjust_the_wp_menu', 999 );
function adjust_the_wp_menu() 
{
    remove_submenu_page( 'themes.php', 'widgets.php' );
}