CSS drop down targeting

I have a drop down menu:

<ul id="nav" class="clearfloat">

	<li class="cat-item"><a href="#">Categories</a>
		<ul class="children">
			<li class="cat-item"><a href="http://xxx.com/">All categories</a></li>
			<li class="cat-item cat-item-6"><a href="http://xxx.com/?cat=6">Some category</a></li>
			<li class="cat-item cat-item-8"><a href="http://xxx.com/?cat=8" >Some category</a></li>
			<li class="cat-item cat-item-11"><a href="http://xxx.com/?cat=11" >Some category</a></li>
			<li class="cat-item cat-item-9"><a href="http://xxx.com/?cat=9" >Some category</a></li>
			<li class="cat-item cat-item-7"><a href="http://xxx.com/?cat=7">Some category</a></li>
			<li class="cat-item cat-item-4"><a href="http://xxx.com/?cat=4" >Some category</a></li>
			<li class="cat-item cat-item-10"><a href="http://xxx.com/?cat=10" >Some category</a></li>
			<li class="cat-item cat-item-5"><a href="http://xxx.com/?cat=5">Some category</a></li>
		</ul>
	</li>
</ul>

And CSS like this:

#nav {
	width:120px;
	background-color:#000000;
	margin-left:0px;
	border:1px solid #000000;
	border-radius:3px;
	letter-spacing:2px

}

#nav > li:first-child{
	text-transform:uppercase;
	font: bold 10.5px Georgia, 'Times New Roman', Times, serif;
	margin-left:-10px;
	margin-bottom:10px;
	display: block;
}

#nav li{
	display:none;
	font: bold 9.5px Georgia, 'Times New Roman', Times, serif;
	text-align:center;
	padding:4px 0 4px 0;
	margin-left:-38px;
}

#nav ul ul {
	display: none;
}

#nav ul li:hover > ul {
	display: block;
}

When hoverin on the Categories title the category list won’t appear. So obviously that is not targetet right? How do I target the categories to show?