For the tots, I'm thinking more along the lines of KidPix (http://www.hotornot.com/r/?eid=OSHMEZE-WBU). Wonderful stuff, and easy enough for a three-year-old to grasp and run with.
For that matter, OpenOffice (or Word, or whatever) is overkill. Something clean like Pages would be much better.
The problem, of course, is cost. Even if OS X was free, most of the interesting, kid-friendly software is not, and wouldn't be. So you'd be running Gimp and OpenOffice on OS X, which would be more painful and just as intimidating as on any other *nix.