Open Office 4 Kids is good for adults, too
The target age group for Open Office 4 Kids is 7-12, but after trying it out, I think there are a lot of adults who would prefer it over MS Office or the full version of OpenOffice. It's available on whichever OS you've set your kid up with: Mac, Windows and (of course) Linux.
Less UI clutter helps make OO4K super-fast. Load times for the app and for creating a new document blew away Office 2008 on my Mac, and I didn't experience any of the annoying lag in loading font menus that I sometimes get in Office.
OO4Kids also has large, very obvious buttons for all of the most common functions of each Office app. The "4Kids" branding is almost a shame, because this could also be Open Office 4 Your Mom Who Always Asks You How To Do Stuff in Office.
On the other hand, I'm glad that any 7-to-12-year-olds who use this app will grow up understanding that Microsoft Word isn't the only way to write a paper.