react.dev/tips/17-children-undefined.md

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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:

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.
    console.log(this.props.children);
  },

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

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

For a more sophisticated example, refer to the last example on the front page.