I run a Cthulhu game based on Mary Poppins, with a touch of Narnia. The players are all older children (10-14), and we're a couple of sessions in now.

Recently, my players discovered that their nanny has been participating in dark rituals with a street performer. Now, the characters saw part of one of the rituals, and one player realized that the ritual was inspired by a movie we've both seen (yeah, dumb of me), and the player decided that killing the nanny is the only way to stop the ritual. So this player announced that his character (an 11-year-old girl) goes to their father's gun cabinet, gets his rifle, and is going to wait for the nanny to come home, so she can shoot her.

What can I give them a spoonful of to help this meta-sin go down?