my kids doc is all for it actually. especially given my families history of heart disease and obesity. If the kid is breast feeding, just start taking 5 - 7 gms a day. thats what my wife did. My daughter has been taking a gram a day for most of her post breast feeding life.
as for increased IQ. my kids are smart, but there is a lot more to intelligence then popping a pill. So its hard to say, but it certainly does not hurt. A severe deficiency can cause a lower IQ as well as obesity and ADHD.
i do too. Vit D is pretty easy. I give it to them in a drop as a reward for cleaning the house. same thing here, the kids doc said we should be doing it.