If you cut magnesium, be very careful with the shavings. If you decide to cut it, you should do so outside. If someone accidently ignites the magnesium, water or a standard fire extinguisher will not work to put it out. Actually, magnesium burns hot enough to split the hydrogen and oxygen atoms in water, resulting in a real interesting situation. To put it out requires a class "D" fire extinguisher using special chemicals. You can extinguish fires of limited size with DRY sand. It should be safe enough if done outside, so long as your scouts don't get any on their clothes.