When you're building your React components, you'll probably want to access child properties of the markup.
Parent can read its children by accessing the special this.props.children prop.this.props.children is an opaque data structure: use the React.Children utilities to manipulate them.
https://facebook.github.io/react/docs/multiple-components.html
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.
https://facebook.github.io/react/tips/children-undefined.html
Type of the Children props
Usually, a component's children (this.props.children) is an array of components. However, when there is only a single child, this.props.children will be the single child component itself without the array wrapper. This saves an array allocation.
https://facebook.github.io/react/tips/children-props-type.html
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>React Lesson 6: Accessing Child Properties</title>
<link rel="stylesheet" href="../bower_components/bootstrap/dist/css/bootstrap.min.css"/>
</head>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.12.2/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.12.2/JSXTransformer.js"></script>
<script type="text/jsx">
/** @jsx React.DOM */
var App = React.createClass({
render: function(){
return (
<BButton>I <BHeart></BHeart> React</BButton>
);
}
});
var BButton = React.createClass({
render: function() {
return (
<a className="btn btn-primary">{this.props.children}</a>
);
}
});
var BHeart =
React.createClass({
render:function(){
return <span className="glyphicon glyphicon-heart"></span>
}
});
React.render(<App />, document.body);
</script>
</body>
</html>
App has two children BButton and BHeart, all thoes chilren come thought from {this.props.children}.
If you don't have {this.props.children}:
var BButton = React.createClass({ render: function() { return ( <a className="btn btn-primary">No passed in!</a> ); } });
We end up with this: 
[Notice:] Just remeber when give class to the render elements, we need to use 'className' not 'class'.