I worked extensively with carbon fiber both as tubing and as sheets. When it cracks/breaks/fails, it becomes razor sharp. If however (as mentioned by ... dang it doesn't scroll up enough for me to quote someone) it's wrapped in something else it would be far less likely to fail catastrophically ("gee, I'm sorry it took your arm off, it was wrapped in foam..."). Although wrapping it up extensively would offset any lightweight advantages, so you'd have to design with some degree of fail-safe and still maintaining a level of fun (what, the 20 pound foam sword isn't heavy enough?).
I digress though, good job on reinforcement. Next we'll have classes on spinning the sword so violently so as to block all incoming foam. Muhahaha, wouldn't that be an awesome sight (or not)?
I to have worked with carbon tubing of the type I posted in my link, and when it has cracked it has never become "razor sharp". Rather I've seen it look more like a pulverized fiberious vine. So I don't know what you are talking about. This is not epoxy coated carbon fiber, which might be what you are used to working with.
Wrapping it in a layer or two of electrical tape or inserting it into some PEX tubing will hardly add weight at all, we're talking a few ounces here. The another reason I used PEX tubing on my particual project was to allow it to fit in the sword without rattling around so much, as well as to increase safety.
The main reason I have advocated for use of Carbon fiber tubing is the fact that it is far more durable than wood, lighter weight metal, and far less dangerous than either if it breaks. I'm just waiting for Cheez's sword to be thrust into someone hard enough to cause the wood to splinter, break sharp poke through the foam and gouge someone's eyes out. Let's face it is's a toy sword, people, especially teens (cause I know I would have back inthe day) are going to hit harder with it than they should.