Create self-populating menu and add extra divs to the menu layout

Your html structure should look like

<ul class="parent-class">
  <li>Page 1</li>
  <li>
    <ul class="child-class">
      <li> Sub menu item 1</li>
      <li> Sub menu item 2</li>
    </ul>
  </li>
</ul>

WordPress should automatically add an “active” class to the active page then you should be able to style everything with css. Something like

.parent-class li ul {
   Display:none;
}

.Active li ul {
   display:block;
}

.parent-class:hover > .parent-class li ul {
   display:block

}