OK, after we get the boffers done, we can get into more child-friendly swordsmanship with these instructions for how to build a lightsabre with common plumbing parts for about US$33. It looks pretty cool, too. Then we can use these rotoscoping instructions and the Crimson application (designed just for rotoscoping lightsaber effects) to film the kids having a nice healthy duel.
Crimson is OS X only, and has trouble running under Tiger, but it's open source, so I'll try to get in and fix it. Some of the Windows lightsabre rotoscoping tools (yes, it's a whole category of applications now) also add neato features like sound effects and flashes (from the blades touching), which would be fun to add as well.