If you're on Linux, you can use PR_SET_PDEATHSIG to accomplish exactly this goal.
My debugging was not aided by the fact that disabling the code where I set PDEATHSIG had no effect, since someone else's code was invisibly setting it regardless.
Related to this is the double-fork pattern to avoid zombie processes (and a couple other issues) when initiating a daemon process.
The parent process was an event loop based python program whose main function was to manage the creation and deletion of these child processes, and the simplest way to spawn child processes without blocking the event loop is to call fork(2) on a thread pool. My thread pool was triaging the number of worker threads based on demand, so occasionally it would decide a worker was no longer needed, and all the child processes that happened to have been created on that thread would get SIGKILL'd — something you rarely want when using a thread pool!
I didn't want the child processes to die unless the parent process's business logic decided they were no longer needed, or if the parent was itself killed (this latter reason being the motivation for setting PDEATHSIG).
Once I understood why my processes were dying, the solution was simple: make sure the worker threads never exit.