I really think you need to buy a copy of OS X, (maybe 10.3 or 10.4?) for each machine, or a family pack. Downloading the OS violates the EULA.

You should be able to put the disk in as you start it up, then restart holding the C key, which should boot from the disk. On the menu bar at the top you should be able to access Disk Utility and erase the HDD, then reinstall the OS and have a fresh start.

If you can't get the disk tray to open there is actually a manual eject, you need to gently open the flap and look for the little hole-type button, and press that firmly with a straightened paperclip or something similar.