In "The Monkey's Paw," why does the mother think the second wish will make everything alright?

The main point the author wants to make about the second wish is that Mrs. White has never seen her son since his accident at the textile mill, but Mr. White had to go there to identify the corpse. Mrs. White still thinks of her son as the way he was on the morning he left for work. She imagines he will look the same and be the same happy, funny boy he was before. These are simple people, as the author W. W. Jacobs makes abundantly clear. It doesn't occur to her to ask for Herbert to return to them as he was before the accident, and it doesn't occur to her husband to wish for Herbert to return as he was before the accident. For one thing, that would seem like an impossible wish--but is it really impossible? That is the biggest question in the story, and it is never answered. Mr. White has limited faith in the monkey's paw anyway, and merely complies with his wife's demand without really expecting anything of such an utterly fantastic nature to happen. After all, Herbert is not only dead, but he has been decaying in his grave. The author has created a horrible impression in the reader's mind without providing any specific details. The reader is curious but probably doesn't really want to have to see Herbert in person if he were to return from the dead.

"We had the first wish granted," said the old woman, feverishly; "why not the second."

  "A coincidence," stammered the old man.

The old man turned and regarded her, and his voice shook. "He has been dead ten days, and besides he--I would not tell you else, but--I could only recognize him by his clothing. If he was too terrible for you to see then, how now?"

 "Bring him back," cried the old woman, and dragged him toward the door. "Do you think I fear the child I have nursed?"

Mrs. White is guided purely by her emotions. She doesn't think about the possible consequences of the second wish. Mr. White is obviously guided more by his experience and reason. He knows that the second wish she is asking him to make is diabolical. It is attempting to contravene the laws of God and nature. The consequences of making such a wish might be even worse than having Herbert, all mangled and decayed, move back into their little home to live with them. The possibilities are like those suggested by Macbeth when he meets the witches and demands answers to all his questions.

I conjure you, by that which you profess,
Howe'er you come to know it, answer me:
Though you untie the winds and let them fight
Against the churches, though the yeasty waves
Confound and swallow navigation up,
Though bladed corn belodged and trees blown down,
Though castles topple on their warders’ heads,
Though palaces and pyramids do slope
Their heads to their foundations, though the treasure
Of nature's germens tumble all together
Even till destruction sicken, answer me
To what I ask you.      IV.1