Not all iMac G3s can install Tiger. Only those with built-in Firewire can, and only those whose firmware has been updated. That means if your iMac is less than 400 Mhz, your maximum operating system is 10.3.9 but you still need to make sure the firmware is up to date for 10.2 or later.
If your iMac is 333 Mhz or less, you'll have to deal with partitioning on Mac OS X because 8 GB and larger hard drives can't install Mac OS X without partitioning.
400 Mhz iMacs can install Tiger if you first make sure that they have at least 512 MB of RAM and 5 GB of free disk space. You'll want to inform your Mom not to let hard disk space to get under 2 GB free for 10 GB hard drives, and 10 GB free for 40 GB hard drives, and follow the 15% arbitrary rule for larger hard drives. If it is 400 Mhz or faster Mac with only a DVD-ROM drive, there is a media exchange program at http://www.apple.com/macosx/upgrade/ for a CD version. Be sure to only use the retail version of Tiger which looks like this and not gray Tiger installer disks. The firmware update must be installed while Mac OS 9 is booted. If you don't have Mac OS 9 installed, you'll have to find the retail OS 9 CD (looks like ) that will install on that Mac before installing 10.2 or later and the firmware update.
Use this article to determine which OS 9 can install on your iMac:
http://docs.info.apple.com/article.html?artnum=25517
Use this article to identify your iMac:
http://docs.info.apple.com/article.html?artnum=58669
This article tells you if you have Firewire on your iMac:
http://docs.info.apple.com/article.html?artnum=58207
And this article tells you the available firmware updates:
http://docs.info.apple.com/article.html?artnum=86117