Don't you have to pass an integer value for the -usize option? If I do not, my mpiexec invocation fails.
BTW, If you pass the universe size to mpiexec, then maybe you should get back the value use maxprocs=usize-1
usize = MPI.COMM_WORLD.Get_attr(MPI.UNIVERSE_SIZE)
You copied&pasted your code rather than attaching it, so maybe this is not in your script, but anyway...
You are missing the `from mpi4py import MPI` stanza in the child code.
After fixing the two issues above, your code runs just fine in my workstation.