If this is very crucial to you, have your script fork, and then use the parent as your worker and the child as the killer. Since the child will die if the parent does, the PPID will always be consistent.

You could also be notified with SIGCHLD whenever your child exits, if you do it in reverse.

I don't know of a way to do what you want reliably, but there are many other ways to get the same results.