SO - 22030784

by Arun P Johny

HTML

<div id="main">
    <div>1</div>
    <div>2</div>
</div>

JavaScript

var el = document.getElementById('main'),
    count = 0;
for (var i = 0; i < el.children.length; i++) {
    if (el.children[i].nodeName == 'DIV') {
        el.children[i].className = ++count;
    }
}