1. The "reasonable person standard" is one that is well established in common law systems, including most of the US, most of Canada, and England ("Reasonable Person Standard" search results on Google). While some have criticised the standard on various grounds, no realistic standard has been proposed to take its place. From Wikipedia: The reasonable person is not the average person: this is not a democratic measure. To predict the appropriate sense of responsibility and other standards of the reasonable man, ‘what is reasonable’ has to be appropriate to the issue. What the ‘average person’ thinks or might do is irrelevant to a case concerning medicine, for example. But the reasonable person is appropriately informed, capable, aware of the law, and fair-minded. Such a person might do something extraordinary under certain circumstances, but whatever that person does or thinks is always reasonable. The "reasonable person standard" is based on "common sense", and common law courts do not generally have a problem with defining it in this manner. The reasonable person standard is often used in criminal child abuse cases, including in the Florida criminal code - see links below (they call it "prudent person" - it's the same thing).

2. What you refer to as "children's welfare department[s]" have a checkered recent history in the US. In a number of prominent cases in the past ten years or so (at least in the Northeastern US, where I live), the negligence of the child welfare workers in improperly investigating and dismissing allegations of negligence has led to the convictions of parents, the dismissal of workers, and the overhaul of such systems, unfortunately not always for the better. Time and time again, it has been shown that for various reasons (of which overwork, lack of funds or education and improper hiring are a few), the actions and determinations of the child welfare agencies have been woefully inadequate and improper.

3. As for "History does not necessarily create a precedence" (I assume you mean precedent) - historically, the common law system is based entirely on history (unlike the civil law system or a mixed system, which includes Scotland), although in modern times common law has been influenced by civil law in the promulgation of various codes, including criminal codes. Despite such codes, though, common law systems retain a strong sense of history. In the US, the primary effects of history in case law are binding precedent and stare decisis. Legally speaking, a case decision always creates precedent in common law systems (unless the judge specifically states in the decision that it does not); even judges issuing decisions that do not appear to follow history must explain his or her reasons behind the differences in the decision in historical terms (what is called "distinguishing" a case). It's true that jurisdictional boundaries in the US are myriad and complex, and so what may have happened in, say, New Jersey, will have little or no effect on a case in Florida. However, as cases pile up over the years, it becomes more and more likely that attorneys can convince a judge to consider cases that would normally be outside of precedental boundaries.

In the past twenty years or so in the US, there have been a number of high profile cases that bear a similarity to this one. Child welfare laws, adequacy of child welfare agencies, and parental rights involving care requirements and diet or religious freedoms are deeply contested areas of law in the US right now, and slowly, there have been changes. I am not familiar with Florida child welfare laws (beyond a dim awareness of a few cases), but relevant past cases there will have some effect on this one, and this case will have some effect on future cases.

Finally, I do not think from the facts available thus far in the case that it can be succesfully argued that the parents provided adequate care - whether or not the child had a birth defect, and regardless of the beliefs of the mother. Intent is not a requirement of one of the charges against the parents, aggravated manslaughter of a child, which is defined in Florida as causing the death "by culpable negligence." Culpable negligence is defined by Florida courts as "exhibiting reckless indifference or gross careless disregard". (Neglect of a child is defined here in Florida Statutes.) Feeding a vegan diet to an infant is not, in and of itself, culpably negligent (as much as many of us may disagree with it). This is a separate issue from the effect of the diet on the child.