Windows 7 has extra network security options, and one of them sets its discoverability. By default in Windows 7, when you connect to a new network you are prompted for what type of network it should be: Public, Work, or Home. These profiles set up the firewall and security options when on that network; I can't off the top of my head list all the things they specifically set up, but when you select Public, sharing and discoverability are locked tight, whereas in Home they are more relaxed.
It's very likely that you have set your network type to Public on your Windows 7 computer. If you open the Network and Sharing Center, under the name of your active network it should say either "Home network", "Work network" or "Public network". If it is indeed set to Public (or Work), click it and then click Home network. You shouldn't have any problems after that.
Alternatively, you can open the start menu and type Network and hit enter. This opens the Network window, which lets you browse other computers on your network. As soon as the window opens, a yellow bar should appear at the top informing you that network discoverability is turned off; click the bar and then click one of the resolution options. This may fix the problem as well.
I haven't had any problem in OS X connecting to my Windows 7 computers that are set to Home network, as they should be. But a Windows 7 computer set to Public network should be basically undetectable.
I tried some development to a remote Mac once. It was a terrible experience. Even over a LAN, the latency of the display update speed was maddening in what should be an instant-feedback development UI.
You could maybe try using virtual machines on the Mac Mini (I don't know how this would affect licensing, you'd probably have to buy another OSX per virtual machine) which would get around not being able to run more than one iOS Simulator as mentioned in the previous answer). While Xcode is a bit of a resource hog I think you could get away with it if its just two users.
I agree that a Mac Mini per developer is the sanest solution for your wallet and the kindest one for your developers though.
Best Answer
Follow this tutorial to configure the windows machines: http://www.howtogeek.com/109655/how-to-remotely-shut-down-or-restart-windows-pcs/
Then install samba: http://forums.boxee.tv/showthread.php?t=38629
Then use the net command to execute the shutdown command just like from linux: http://www.commandlinefu.com/commands/view/2330/shutdown-a-windows-machine-from-linux
If that works you can make a sh file with the command repeated for all the ip addresses of all the computers.