Custom CSS class or ID on menu items that have a Submenu

You could use jQuery, if you’re open to a jQuery solution?

<script type="text/javascript">  
jQuery(document).ready( function($) {
    $('#your_menu_id li').has('ul').addClass('has_children');
});
</script>

Slight modification of what i posted here basically.

Any menu item with a submenu will then have a has_children class you can target in the stylesheet..

#your_menu_id li.has_children { /* your styling */ }
#your_menu_id li.has_children ul { /* your styling */ }
#your_menu_id li.has_children ul li { /* your styling */ }

Hope that helps..