This came up today as something that is a very basic requirement when developing complex forms, but there was no way built in to Yii to handle it quickly and consistently across multiple models, so I created a custom validator for it. I hope it helps others to move forward quickly with their complex form development ;)
The need was that when a specific field was set or a specific value was selected for a field, additional fields would become required.
The solution? ChildrenRequiredValidator
Any feedback or suggestions are more than welcome!