Make one last TimeMachine backup of each computer. Start a computer with the Snow Leopard (10.6) system disk (I assume you have the commercial release, the disk with a Snow Leopard on it) and its TimeMachine drive connected. To boot with the SL system disk, turn the computer on and immediately press & hold the 'C' key when you hear the chime. Once you see the Apple logo you can release the key.
After choosing your language you'll be at the installation screen. In the menubar select Utiltiies, and choose Disk Utility. Select the computer's hard drive and choose 'Erase'. Quit Disk Utility when it finishes erasing (it shouldn't take long) and you'll be back to the SL install window. Do it. This step will take 30-60 minutes. Eventually the installation will be completed and you'll be asked a few questions. Eventually you'll come to the Setup Assistant.
Read this article about Migration Assistant. SA and MA are basically the same thing, they are different in that you get one chance to migrate your account(s) and software with the SA - after you've installed the new operating system or started your Mac for the first time. You can run MA at any time but there are reasons why you are best off running the SA.