Fijate algo así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
</head>
<body>
<ul id='myul'>
<li>a</li>
<li>b
<ul>
<li>b1</li>
<li>b2</li>
<li>b3</li>
</ul>
</li>
<li>c</li>
<li>d</li>
</ul>
<script>
el=document.getElementById('myul');
els=el.getElementsByTagName('li');
vec=[]
for(i=0;i<els.length;i++){
if(els[i].parentNode==el)
vec.push(els[i]);
}
alert(vec.length);
</script>
</body>
</html>