Adding a unique class to wp_nav_menu

How would I go about adding a unique class to the direct child li elements of the main parent ul element generated with wp_nav_menu?

<nav id="access" role="navigation">
  <div class="menu-main-container">
    <ul id="menu-main" class="menu">
      <li id="menu-item-30" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-30"><a href="http://www.site/page1-2/">Page1</a>
        <ul class="sub-menu">
          <li id="menu-item-50" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-50"><a href="http://www.site/sub-page1-1/">Sub-Page1-1</a></li>
          <li id="menu-item-49" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-49"><a href="http://www.site/sub-page1-2/">Sub-Page1-2</a></li>
        </ul>
      </li>
    </ul>
  </div>
</nav>

I’d like to add the class ‘menu_element’ to ‘li id=”menu-item-30″‘ and every child li of the parent ul after. Is this possible?

Thanks in advance!

[ Please do not bump, it’s not permitted here. ]