Train a child up, from an infant by feeding it healthy food. Not that canned junk you buy off-the-shelf or stuff in little tiny jars, you got to actually cook vegetables and mashed them yourself and sources of protein. I say sources of protein other than meat because there are many sources of protein. And that’s where you’ll find the B12.
And if the mother is nursing the mother needs to eat healthy during the nursing cycle. I haven’t kept up with pediatric methods, since I no longer have children living in the home but I will say that I’m pretty sure wet-nursing is the preferred feeding regimen for infants these days. But it’s been awhile since I really read anything concerning that.
Don’t drink alcohol and breast feed. I don’t care what the charts show how many hours it takes to get out of your system. Those charts say 2 hours but that is WRONG. I’ve seen it first hand when the mother breast feeds the next morning after partying and the poor baby gets zoned out.