"Bring him back," cried the old woman, and dragged him toward the door. "Do you think I fear the child I have nursed?" (3.25)
Have you ever wanted something so badly that you ignore the risks and just focus on the possible benefits? Mrs. White misses her son so badly that she doesn't think about the fact that his body was, as Mr. White points out, horribly mangled and has been dead for ten days. If Herbert comes back like that, he may be pain. If you've seen any zombie movies, you know he'll also probably be murderous and maybe even hungry for human brains.
"You're afraid of your own son," she cried, struggling. "Let me go. I'm coming, Herbert; I'm coming." (3.45)
Mrs. White isn't being fair here, and Mr. White is able to see clearly enough to recognize this. He has learned his lesson about wishing hastily. He can see all the problems that undead Herbert might bring.