How to Prevent Sub List to Slide Down and Up Parent List

How to Prevent Sub List to Slide Down and Up Parent List

Can you please take a look at This Demo  and let me know how I can stop /prevent the sub menus to slide up the list. If you click the  Two  or Three you can see there are sub menus as  Two _ 1 .. Two _ 4  and Three _ 1 to Three _ 4 now I would like to stop them sliding up the opened  musthidden class.
Here is the code I have:
HTML
  2. <ul id="slider">
  3.     <li class="expandOnClick"> <a href="#">One</a></li>
  4.     <li class="expandOnClick"> <a href="#">Two</a>
  5.         <ul class="musthidden">
  6.             <li><a class="prevent" href="#">Two _ 1</a></li>
  7.             <li><a class="prevent" href="#">Two _ 2</a></li>
  8.             <li><a class="prevent" href="#">Two _ 3</a></li>
  9.             <li><a class="prevent" href="#">Two _ 4</a></li>
  10.         </ul>
  11.     </li>
  12.     <li class="expandOnClick"> <a href="#">Three</a>
  13.         <ul class="musthidden">
  14.             <li><a class="prevent" href="#">Three _ 1</a></li>
  15.             <li><a class="prevent" href="#">Three _ 2</a></li>
  16.             <li><a class="prevent" href="#">Three _ 3</a></li>
  17.             <li><a class="prevent" href="#">Three _ 4</a></li>
  18.         </ul>
  19.     </li>
  20.     <li class="expandOnClick"> <a href="#">Four</a></li>
  21. </ul>
jQuery:

  2. $(".expandOnClick").click(function (e) {
  3.     e.preventDefault();
  4.       $(this).siblings().children(".musthidden").slideUp();
  5.       if($(this).children(".musthidden").css('display') == 'none') 
  6.        {         
  7.           $(this).children(".musthidden").slideDown();}
  8.        else
  9.        {
  10.          $(this).children(".musthidden").slideUp();
  11.        }
  12.  });
and CSS:

  2. .musthidden {display:none;}
  3. ul {list-style-type:none;}
  4. li {background-color:#2d2d2d;}
  5. li:hover {background-color:#ccc;}
  6. a { color:#fff;}

Thanks