Thanks guys!! I haven't gotten my dress yet. I tried it on in white and ordered it in ivory. So I am not sure how dark the ivory is.... And I want to order the flower girl dresses from macys this weekend. They are having a great sale on girls dresses and an extra 20% off, making them about $24-40 a dress!!!
It depends-- Your dress will look dingy when you are standing with your flower girl, in pictures with your flower girl, etc. If you're okay with that, then yes, its fine. But it really will just look like your dress is dirty (and not that they are two different shades of white).
What Barbara said. With smaller things, like the men's shirts, this isn't such a big deal, because you're going to have the suit/tux jackets separating the white shirts and the ivory dress. In portraits however, the flower girls are often situated in front of the bride and groom, and therefore their dresses will be in direct contrast with yours, making yours look dingy or yellowed. If you can get them in ivory, by all means do so!
Edit: My dress is ivory, and against the white garment bag, still looks pretty white. The dresses however, will probably be the stark, bright white (tends to look slightly bluish in bright light) which WILL make the ivory stand out.