Hi Steven,
Sorry for the inconvenience caused. In the present scenario of Diagram control we have no special click events for the Child nodes of the complex symbol. The Node_Click event gets fired when you click on the complex symbol. However I have logged this as a feature request #1313. You can track the status of this feature here:
Feature Request 1313 - Add support to access the child nodes of the complex symbol.We will try our very best to implement this feature and update you once this feature is implemented.
Please let me know if you have any questions.
Best Regards,
Nagaraj