The breakdown products of the Miracle-Mineral-Supplement should not cause any problem with your child but it might make the breast milk taste differently as anything a women eats will, like onions or garlic.
You did not mention if your wife had any health problems. And what are her plans or desires as to continueing breastfeeding.
It is possible that anything that changes might decrease her ability to produce enought breast milk, so if she does start on it, and the amount decreases just stop it. Miracle-Mineral-Supplement is all out of a persons system in 12 hours.