Even though Nick was a thief, he still liked to think of himself as a good person. He didn't steal from the poor, and he never dirtied his hands with the fairy corpses that some rich men collected like butterflies. The new fairy saw the young peasant lad currently shouting for his father, and realized that perhaps not everyone was so scrupulous.
She also realized that the boy was holding back, a little afraid of the fairy ring himself, and that she had some time before the father arrived. But what to do? There was nothing around that looked like a weapon for a fairy, and she imagined that the father and son had a procedure for dealing with trapped fairies. Ugh, humans, she thought, and then wondered where that thought had come from.
OK, mushrooms. Dig up one of the mushrooms and see if that breaks the circle? She was fairly sure that if she broke the trap, she could just fly away. Picking a spot on the other side of the dead deer where the boy couldn't see what she was doing, she began to work.
Of course, uprooting a mushroom would have been child's play for Nick, but for a tiny fairy it was the equivalent of pulling a bush out of the ground. Niki sweated and strained, and made a little progress. Then she heard the boy, who had moved to see what she was doing, shout "You can't do that!"
Good, she thought, they weren't expecting that.
"When my dad gets here you're going to be in trouble!"
Even better, Niki thought, people didn't make threats unless they felt threatened. She felt the mushroom give way.
The magic left the circle in a mighty surge that carried Niki into the air. As she looked down, she saw that the boy had been transformed by the departing magic into a giant mushroom, and that an angry older peasant, the boy's father no doubt, had arrived on the scene and was vehemently cursing. Oh well, perhaps the old man wuld find Nick's gold, that Niki had to leave behind, some consolation for the loss of his son and his fairy trap.