React onClick event on component

This depends on your SensorItem component’s definition.
Because SensorItem isn’t a native DOM element but, like you said, another React component, onClick as defined here is simply a property of that component. What you need to do is, inside of the SensorItem component pass the onClick prop to an DOM component’s onClick event:

var SensorItem = React.createClass({
  render: function() {
    return (
      <div className="SensorItem" onClick={this.props.onClick}>
       ...
      </div>
    );
  }
});