"Well-planned vegan and other types of vegetarian diets are appropriate for all stages of the life cycle, including during pregnancy, lactation, infancy, childhood and adolescence."
http://www.eatright.org/cps/rde/xchg/ada/hs.xsl/advocacy_933...
Can you at least offer a source for your shocking claims?
Simply Googling for "vitamin B12 brain development" will return you any number of papers on the subject, but most of them are behind their respective journal's paywalls unfortunately. However, the fact their abstracts include works like "cerebral atrophy" might give you some idea of their content.
Cobalamin is added to any number of foods (usually in the form of cyanocobalamin), including bread, cereal, orange juice, soy milk, veggie burgers, soft drinks, and on and on. It is produced by bacteria.
I know multiple mothers who have breastfed while on vegan diets. It is trivial to get B-12, simply by taking a multivitamin, which they should do anyway regardless of diet (starting with prenatal preparation including folic acid supplementation).
I would suggest doing more research on the subject of veganism and discontinuation of spreading further misinformation.