We also have managed to stay relatively illness free, largely because of vitamin D. You're right to question the dosage - I find that any more than 2,000 IU causes a massive allergic reaction, as described in my blog:Vitamin D overdose?
But, hallelujah! The bronchitis, colds and flus have been a thing of the past for years now.
Yes, I'm afraid to take that much and that's for adults of average size. Being 4' 11" and 100 lbs, I've found even the adult dose of medication can be too much for my size. Thus, I learned very early to question dosages. No medication or vitamin is a one size fits all deal, not even for adults. One day a week, I take a day off from my vitamins too, just to make sure I'm getting an overload, so that 1400/day is only for 6 days a week. I can only hope that respiratory infections are a thing of the past for me, because I've spent a lifetime dealing with them.