According to the Oxford English Dictionary, an obsolete use of the word "breed" means "to generate." The text is saying that cooked rabbit's brains are good to feed children in order to generate good teeth. Embedded in this is not only the obsolete meaning of "to breed" but also a folk belief. Europeans often felt that various rodents - including rabbits, mice, etc., - were particularly good at growing teeth, and that they could impart that talent on children through various means - in this case by ingesting cooked brains. Other times, there were approaches that used sympathetic magic, calling on the ability of the rodent by other means that had less of a direct, nutritional virtue. That said, feeding the nutritional-rich brains to developing children may have been practical advice, but I am not a nutritional expect, and I don't even play one on TV.