Try
var el = document.getElementById('elem'),
//modern browsers IE >= 10
classList = 'classList' in el;
for (var i = 0; i < el.children.length; i++) {
var child = el.children[i];
if (child.tagName == 'DIV') {
if (classList) {
child.classList.add('test');
} else {
child.className += ' test'
}
}
}
Demo: Fiddle