666FoX666,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.green {
color: #008000;
}
</style>
</head>
<body>
<div id="test">
<h1>First Child</h1>
<p>Last Child</p>
</div>
<div class="test2">
<h1>First Child #2</h1>
<p>Last Child #2</p>
</div>
<div class="test2">
<h1>First Child #2</h1>
<p>Last Child #2</p>
</div>
<script>
[].forEach.call( document.querySelectorAll('.test2'), function(el) {
el.querySelector('p').classList.add('green');
//el.children[1].style.color = 'green';
});
</script>
</body>
</html>