B.M.
Hi Meghan,
As the previous mom stated, many of us have vitamin D deficiencies. In the summer, its not an issue. 10-15 minutes outside will give us the adequate amount of vitamin d that we all need. In the winter, when the sun isn't as plenty, a good supplement (a good multi-vitamin) is what we should take. Not all vitamins are created equally, so you want to make sure you are chosing a vitamin that has a strong science and research behind it.
I can offer you some information on how to go about looking for that. But you can do some research yourself, by contacting the company and asking about their science and research.
The other question you asked about is your daughter. Vitamin D is not passed through breast milk, so even if you did not have a deficiency, your daughter would need a supplement as well. There are a lot of children's vitamins with a ton of sugar and cannot guarantee that what is on the label is actually in the bottle.
I'd be happy to share some information with you, if you are interested.
B.