This is an issue with IDLE, which you're using to run your code. IDLE does a fairly basic emulation of a terminal for handling the output of a program you run in it. It cannot handle subprocesses though, so while they'll run just fine in the background, you'll never see their output.
The simplest fix is to simply run your code from the command line.
An alternative might be to use a more sophisticated IDE. There are a bunch of them listed on the Python wiki, though I'm not sure which ones have better terminal emulation for multiprocessing output.