Interesting.
There's something weird with the first child:

=== Child #1 =========

Thread 0x00000445:

Thread 0x00000444:
  File "/home/haypo/cpython/Lib/threading.py", line 237 in wait
    waiter.acquire()
  File "/home/haypo/cpython/Lib/threading.py", line 423 in wait
  File "/home/haypo/cpython/Lib/threading.py", line 685 in start
  File "/home/haypo/cpython/Lib/multiprocessing/queues.py", line 187 in _start_thread
  File "/home/haypo/cpython/Lib/multiprocessing/queues.py", line 107 in put
  File "/home/haypo/cpython/Lib/concurrent/futures/process.py", line 168 in _add_call_item_to_queue
  File "/home/haypo/cpython/Lib/concurrent/futures/process.py", line 206 in _queue_management_worker
  File "/home/haypo/cpython/Lib/threading.py", line 690 in run
  File "/home/haypo/cpython/Lib/threading.py", line 737 in _bootstrap_inner
  File "/home/haypo/cpython/Lib/threading.py", line 710 in _bootstrap

See the last thread created (0x00000445)?
0x00000444 bootstrapped 0x00000445, and is waiting for it to signal that it's running.
Since there's no backtrace for this thread, it means that it's stuck inside t_bootstrap or really early in the call stack (or didn't start at all?).
You don't have a coredump, do you?
I guess it's not reproductible either?