A/N: Perposterice. Title/prompt is from a list of eggcorns (Google it), and was part of a challenge. More crack. Enjoy!
It Doesn't Pass Mustard
Draco's scream brought his mother running.
"Darling, what is it? Oh, stop having hysterics, you'll frighten my granddaughter."
Draco flapped his hands frantically at the child in question, secured to the changing table with a charm. "That is not a normal baby! It can't be!"
Narcissa sniffed at her son's antics. "You're a father now, and it's time you got past your delicate sensibilities, Draco. Babies soil themselves. Yes, even Malfoy babies. I am this child's grandmother, and I am not required to do the dirty parts any longer. I am only required to spoil Malia and then give her back to you after I have fed her enough chocolate to sustain a small city for a year."
"Mummy, you don't understand. Malia has somehow managed to pass mustard. It's… it smells like it!"
Narcissa sighed. "Your daughter, Draco, though certainly destined to be a most remarkable witch, does not excrete mustard. Now get over there and clean her up. Don't forget the powder when you put the clean nappy on." She paused, and added darkly, "Do not tell your wife the baby passed mustard, Draco. I will not humor her if she hears of this."
Draco steeled himself and approached the baby cautiously. It was difficult, being the son of Narcissa Malfoy, and the man who'd managed to get Luna Lovegood to settle down.