Also, depending on where you live you may never need to supplement D, if your LO gets adequate sun exposure. We used to give vitD during Michigan winters but stopped when we moved south and started playing outside 30+ min everyday.
I second the vitD only drops, or at least the Tri-Vi-Sol (which is better tasing; my ped actually recommended not using the Poly-Vi-Sol because the extra vitamins in there are not necessary for a breastfed baby and the iron in it tends to cause constipation)
Mama to M, BF 17mo
and H, born Feb '12