Do you know your vitamin D level? Even in spite of eating healthy and getting a good amount of sun (well, as best as I could in a part of the US with lots of snow in winter), my vitamin D level was profoundly low. So low, in fact, that it didn't register on the lab equipment. It may be worthwhile to get a baseline level to know what your starting point is so that you can supplement with the best information you can have in hand. I had to start off by taking 50,000 IUs of vitamin D3 (and it's important to take vitamin D3, not D2 which is the more common form found in supplements because it's cheaper and more readily available) 3 times a week.
Some other influencing factors are your degree of pigmentation (lighter skinned people absorb vitamin D more readily through the skin), how much sunscreen you use (as sunscreen blocks the absorption of vitamin D), and take into consideration that the synthetic vitamin D that is added to pasteurized milk is not readily bio-available so it's actually not a good source of vitamin D.
Vitamin D is a hormone, not a vitamin and can be toxic if you get too much of it (which is why I think it's good to have a baseline).
All that being said, I give my 7yo 800IUs a day and my 3yo 400IUs a day. I take 50,000IUs a week (but my most recent level was 52 which I still consider insufficient but not deficient). The amount of supplementation varies according to the time of year (availability of sunlight). I also have our levels checked to keep us between 80-100 (over 120 is indicative of toxicity; mainstream medicine considers over 36 to be the low end of normal [it varies] while alternative medicine practitioners will consider a range of 70-100 to be optimal).
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