what argument does fern use that convices her father not to kill the runt pig

Fern emotionally pleads her case to her father. This is a difficult task because farm life isn't always consistent with sentimentality over animal husbandry (bringing up animals for consumption) Still, Fern is young and she uses the best argument that she knows,

"But it's unfair," cried Fern. "The pig couldn't help being born small, could it? If I had been very small at birth, would you have killed me?"

Her father is a practical man but he also sees his daughter’s convictions; to her this is a very plausible argument. He allows her to nurse the pig.