You're implying that if you use fork (i.e. create child processes), the workers won't be in the same process group. That's not true. A process group is a group of processes, not a group of threads. (Mind you, all your threads will be in the same process group by virtue of being in the same process.)

if (!fork()) { sleep; exit(0); }
if (!fork()) { sleep; exit(0); }
kill TERM => $$;   # Only kills self
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if (!fork()) { sleep; exit(0); }
if (!fork()) { sleep; exit(0); }
kill -TERM => $$;   # Kills all three processes
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