TexasCHLforum.com

I'd guess:
Guest in house knew there was a "gun" and where it was kept (maybe on piano, maybe not).
Guest brought "gun" out to show off.
Other girl said "you shouldn't have that out here."
Guest pulls trigger, saying, "it won't go off, see."

As far as teaching kids about firearms, movies have far more reach than aware parents. Very few movies (including a Marx Bros. movie!) show indexed fingers not continually on the trigger, so it becomes a "no-brainer" to keep the finger on the trigger.