Главный процесс
import os
import subprocess
import sys


child = os.path.join(os.path.dirname(__file__), "./child.py")
word  = 'word'
file = ['./parent.py','./child.py']

pipes = []
for i in range(0,2):
  command = [sys.executable, child]
  pipe = subprocess.Popen(command, stdin=subprocess.PIPE)
  pipes.append(pipe)
  pipe.stdin.write(word.encode("utf8") + b"\n")
  pipe.stdin.write(file[i].encode("utf8") + b"\n")
  pipe.stdin.close()

while pipes:
    pipe = pipes.pop()
    pipe.wait()

=====================================================================
Сhild процесс
import sys

word = sys.stdin.readline().rstrip()
filename = sys.stdin.readline().rstrip()

try:
  with open(filename, "rb") as fh:
    while True:
      current = fh.readline()
      if not current:
          break
      if (word in current ):
          print("find: {0} {1}".format(filename,word))
except :
    pass