They won't be charged, nor should they be. There is no law in Arizona (nor in many other states) against letting responsible teenagers have access to the tools they need to protect the family.
Obviously, the parents made the right call in this instance. If the newspaper account is accurate, it's reasonable to believe that all four kids would very likely be dead, or raped and otherwise traumatized, if the parents hadn't taught their son what to do in such a situation.
My personal website: Cornered Cat