.children

<ul class="level-1">



  <li class="item-i">I</li>



  <li class="item-ii">II



    <ul class="level-2">



      <li class="item-a">A</li>



      <li class="item-b">B



        <ul class="level-3">



          <li class="item-1">1</li>



          <li class="item-2">2</li>



          <li class="item-3">3</li>



        </ul>



      </li>



      <li class="item-c">C</li>



    </ul>



  </li>



  <li class="item-iii">III</li>



</ul>

 

$( "ul.level-2" ).children().css( "background-color", "red" );