Auto-Subscribe, or Custom Mail Config for Child Work Items?

Is there some way to setup email notification based on relationship?

For example:

I own Task 123, which has a Child Task 456 which has been assigned to someone else

I want to be able to be notified, particularly if they close Child Task 456 or modify a date field in Child Task 456 but would be willing to get notification of *all* updates if needed

I don't see this option in the User "Mail Configuration" - even finding a way to make the owner of the Parent Task 123 automatically a subscriber of its child tasks would be helpful