Still learning, but I found solution that satisfies me at least.

1. Create fifo descriptor in parent

if not os.path.exists(fifo_path):
        os.mkfifo(fifo_path)

2. Launch process that will write to this fifo

    python_process = multiprocessing.Process(
        target=child_func,
    )
    python_process.start()
    
   
    write_fd = open(fifo_path, "w")
    write_fd.close()

3. Open fifo and pass to subprocess

      fifo = os.open(fifo_path, os.O_RDONLY)
      fzf = subprocess.Popen(
           [
               "fzf",
           ],
           stdin=fifo,
           text=True,
       )