I'm assuming the computers are not connected to a domain? If not, I doubt the local account on the main Win8 laptop will have the appropriate permissions to shutdown the other machines. My lab is currently all domain connected machines so can't really test but suspect you will need to use PSexec (link) with a command along the lines of:
Obviously you'll need to replace the IP address, username (johndoe) and password with the appropriate cred for you machine(s).
Code:
psexec \\10.0.0.5 -u johndoe -p mypassword -accepteula "C:\windows\system32\shutdown -f"