As noted in issue #11870, making faulthandler capable of dumping child processes' tracebacks could be a great aid in debugging tricky deadlocks involving for example multiprocessing and subprocess.
Since there's no portable way to find out child processes, a possible idea would be to make the handler send a signal to its process group if the current process is the process group leader.
Advantages:
- simple
- async-safe
Drawbacks:
- since all the processes receive the signal at the same time, their outputs will be interleaved (we could maybe add a random sleep before dumping the traceback?)
- children not part of the same process group (for example those who called setsid() or setpgrp()) won't be handled

I'm not sure how this would work out on Windows, but I don't event know if Windows has a notion of child processes or process groups...