The Crystal Code was recited by Jedi during the construction of a lightsaber. We recite it before the beginning of lightsaber dueling sessions.
The crystal is the heart of the blade.
The heart is the crystal of the Jedi.
The Jedi is the crystal of the Force.
The Force is the blade of the heart.
All are intertwined.
The crystal, the blade, the Jedi.
We are one.
This comes from Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham.
During dueling, safety is the first concern. I conduct the duels on a point system similar to point sparring martial arts tournaments. The first one to three touches wins.
First, with kids, keep control of the light sabers or you'll have kids who aren't in the current duel hitting other kids. I recommend having two sabers for each adult you have running a duel, no more.
Set up boundaries for your duel arena. If one of the duelist leaves the area, stop the duel and bring them back to the center. If the same duelist leaves the area twice, warn them, and on the third time you can disqualify them.
Start the action with their light sabers touching. Say, "Ready, Set, Duel." When there is a touch, say, "Stop" and verify the hit and give the current score for the two duelists. Check the lightsabers' foam for safety, especially the filler in the end of the tube.
Don't let the kids hit trees or anything that will tear up the foam.
Don't allow head shots. If there is a deliberate head shot DQ the duelist and don't let them compete again that meeting. If it is accidental, give them a warning. Then DQ them after a second head contact. I haven't seen the need to use helmets yet. Older kids, or adults may want to, but the padded lightsabers and controlled strikes seem to be safe enough. You must teach the kids to stop the strike at the point of contact and not strike through the opponent. Make the duelists hold the lightsaber with two hands. It probably doesn't need to be said, but punches, kicks, grabbing, tackling or other attacks not with the foam of the lightsaber are not allowed. No strikes with the handle of the lightsaber either.
Compared to other kids sports, lightsaber dueling is pretty safe. In soccer and baseball, kids get hit by the ball. Football is far more dangerous. With the foam lightsaber, I think it's even safer than kids karate sparring. Probably more dangerous than badminton or croquet.