this.props.children undefined

You can't access the children of your component through this.props.children. this.props.children designates the children being passed onto you by the owner:

/** @jsx React.DOM */

var App = React.createClass({
  componentDidMount: function() {
    // This doesn't refer to the `span`s! It refers to the children between
    // last line's `<App></App>`, which are undefined.

  render: function() {
    return <div><span/><span/></div>;

React.renderComponent(<App></App>, mountNode);

To access your own subcomponents (the spans), place refs on them.