Well, one obvious one springs to mind. But I doubt it's any simpler to use than PowerPoint.
(Confession: I'm not a big fan of presentation software. And since I have a strong personal dislike for PowerPoint in particular, I'm not the most unbiased opinion out there.)
Inside the Open Office
Suite of apps is something called Impress
which was designed to be a direct FOSS replacement for PowerPoint. I'd suggest they start with the no-install portable version of OpenOffice to see if they like it first. They can always install the regular version onto their hard drive if you later decide to keep it.
OpenOffice Homepage: www.openoffice.org
Get a portable version here: http://portableapps....ice.org_portable_3.1
Although PowerPoint is very popular in the education and business world, there are some people who have raised serious questions as to whether or not all those bullet point presentations may be doing more harm than good. This is a good introductory article (with links) about the debate:http://teachwithpowe...-of-bullet-list.html
Worth a read, even if you end up not agreeing with the PowerPoint detractors.
BTW: I'm not so sure Office 2003 is all that much overkill for grade school kids. They grasp technology a lot more quickly that most would suspect. Presentations require a certain way of thinking, a skill set, and some tools to effectively put one together. As long as a child understands what a presentation is supposed to do, and how to put one together, it's no big deal to get them rolling on PowerPoint. I recently saw a presentation by two local 7-year old kids (Girl Scouts to be exact) that could teach some "corporate communication" types a thing or two about how to get your point across.
Just my 2¢