My kids got lightsabers while we were in Disney, and we have two rules for using them:
1. No hitting anything else except other lightsabers; and
2. You must make the lightsaber sound when swinging your lightsabers!
My son Peter is really into Star Wars right now. The kids briefly watched it with me once about 8 months ago when Betty was away for a few hours, and Peter has never stopped loving Darth Vader since. Peter even got Star Wars bath toys while in Disney. He calls himself a "bad guy" (in a deep voice) because thinks all the bad guys are really good guys. I mean, Darth Vader is super cool, so how could being a bad guy really be so bad? Does not compute!
Anyway, back to the rules of lightsaber duels. The first rule (don't hit anything) doesn't really apply with my son, because he's two. It's a given that he's going to at least accidentally hit something around him (usually me) when he's playing with his sword. Heck, I even want to smash stuff with it when I fence with him, except I'm old enough that those thoughts get squashed the millisecond my brain conjures them. But my son, well, he's taken to that old adage, "It's better to ask forgiveness than permission." Except he doesn't ask for forgiveness. Nor should he, since he's usually in the character of Lord Vader.
We have a routine when we duel with our magic swords of light. First we square off, staring each other down. Then we flick our wrists to expand our lightsabers (all the while making the "brrrzzz" sound, of course). Then he attacks, running towards me and swinging his sword while I go on the defense, until he ultimately cuts off my arm - which just goes into my shirt sleeve - and my lightsaber falls to the floor.
At this point my son shows that he might really be on the light side of the force by helping me to regrow my arm and take back my lightsaber. But maybe he's got a more heinous intent, as he promptly faux-chops my arm back off again.
The only time my son ever really gets in trouble when he's playing with his lightsaber is when he's not making the sound when he swings it. This is non-negotiable. He either makes the sound, or it goes bye-bye.
What, you didn't think I was going to raise a Trekkie, did you? Gotta teach your kids the right way to do things!