only allow children of a specific type in a react component

You can use the displayName for each child, accessed via type:

for (child in this.props.children){
  if (this.props.children[child].type.displayName != 'Card'){
    console.log("Warning CardGroup has children that aren't Card components");
  }  
}

For React 0.14+ and using ES6 classes, the solution will look like:

class CardGroup extends Component {
  render() {
    return (
      <div>{this.props.children}</div>
    )
  }
}
CardGroup.propTypes = {
  children: function (props, propName, componentName) {
    const prop = props[propName]

    let error = null
    React.Children.forEach(prop, function (child) {
      if (child.type !== Card) {
        error = new Error('`' + componentName + '` children should be of type `Card`.');
      }
    })
    return error
  }
}