test.jsx

class Alert extends React.Component {
  render() { 
        return (
          <div className='alert'>alert</div>
        )
  };
}
class Header extends React.Component {
  render() { 
        return (
          <div className='header'>header</div>
        )
  };
}


class App extends React.Component {
  componentDidMount() {
      this.heightHeader = React.findDOMNode(this.refs.header).innerHeight;
    }
  
  render() {
    var heightAlert = 30;
    var divStyle = {paddingTop: 20 + this.heightHeader + heightAlert};
        return (
          <div style={divStyle}>
            <div className='top'>
              <Alert/>
              <Header ref="header"/>
            </div>
          Контент
          </div>
        )
  };
}
React.render(
  <App />,
  document.getElementById('example')
);