RDA for vit D is currently at 400. Unless a mom is taking high dose vit D supplements, she is not getting 4,000 - 8,000 / day.
I frequently supplement with 5,000 / day and am still under the 50 minimum mark.
I give 400 UI to my baby every day starting at 3 months. I also make sure she gets some sun when the sun is out and it's reasonably warm.
Blood tests are the only way to make sure you are sufficient in Vit D.
You can overdose on it, but not likely as we are all mostly deficient.
I personally wouldn't take over 10,000 / day unless monitored every other month with blood tests.
Vit D is crucial for so many body functions. With the media and doctors scaring people out of the sun and into using sunscreen, I am sure vit D deficiency has sky rocketed.