Херней страдаешь.
<div></div>
<div></div>
<div><span></span><span></span></div>
<script>
function query(selector, context) {
return (context || document).querySelectorAll(selector);
}
var divs = query('div');
alert (divs.length);
var spans = query('span', divs[2]);
alert (spans.length);
</script>