[Answered ]-REST API: Delete operation affecting other objects

The propert HTTP status code is 410 GONE.

Indicates that the resource requested is no longer available and will not be available again. This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource again in the future.

There is no special way to informa about the deletion having affected children of the deleted resource. You should document your resources and clearly state that some resource can an will only exist as children of a parent.