class Form extends React.Component {
  _fill(e) {
    console.log(e.target.value);
  }
  render() {
    return(
      <form>
        {this.props.children}
      </form>
    );
  }
}

class Field extends React.Component {
  _setValue(e) {
    this.props.fill(e);
  }
  render() {
    return (
      <input type="text" onChange={this._setValue}/>
    );
  }
}

render(
  <Form>
    <Field name="first_name" />
    <Field name="last_name" />
  </Form>,
  document.getElementById('form1')
);

render(
  <Form>
    <Field name="first_name" />
    <Field name="last_name" />
    <Field name="age" />
  </Form>,
  document.getElementById('form2')
);