This is quickly leaving the realm of this forum, but I'll bite.
If she can't work an iPad, I think it'll also be very difficult for her to use those Linux PCs or a Mac.
The Mac can be made to work in a very basic mode called Simple Finder (as pointed out earlier): »
www.switchingtomac.com/t ··· in-os-x/The main interface becomes a single window with large icons that can be used to access approved apps (Mail, Safari, etc). The downside is that once you get into the app, it's the same full-feature app as on any other computer. The application menu's text will be relatively small (compared to WebTV's menus), navigating will require moving the small cursor around (you could install third party tools to make the cursor larger), etc.
If you reduce the resolution for the TV, everything will be easier to read, but then you'll have to scroll more often to see all the content on a page.
As for security, at least with the Mac solution, you can pretty reliability just turn on the firewall and call it a day. At this point, there are so few viruses/trojans for the Mac that it'd be statistically impossible for anything bad to happen, especially on a limited user account.
If you really wanted to go all out, you could purchase Deep Freeze. It reverts the entire computer to a "frozen" configuration after every boot. You can delete the entire system folder, reboot, and everything will work just fine. Downside with this approach is any saved documents/bookmarks are lost after a reboot.
