We now have a date

Oh dear Glamma difficult for you, caught in the middle.
Agree withShysal it's the bride's choice, I think all you can do is try to make your DD see this and that she will feel much better herself if she can just rise above it all and manage to smile sweetly, rather than harbouring resentment.
Not sure how old your DGD is but hopefully she could be mollified with a lovely new outfit of her own choosing.
What about readings? could your DD be asked to do that?

Oh glamma it's so difficult isn't it? DD is planning her wedding for 2014 and already I am biting my tongue about some aspects of the arrangements. Especially hard when your DGD is upset. I agree that it has to be the bride's choice, perhaps she doesn't want little bridesmaids or perhaps (as in my DD's case) there are so many little girls amongst family and friends that she doesn't feel she can choose just one or two. I'm not sure whether it would be wise to bring this up, only you know the people involved and how you think they might react. And emotions are always running high when weddings are being planned! Xx for you and your DGD

I am afraid it has to be the bride's choice. In some families the bridesmaids always and only come from the bride's side , so do not take it as a slight. Perhaps DGD can choose herself a pretty outfit and you could tell her she will be the best dressed guest.

Have a great day when it comes, and please don't let this situation spoil your pleasure, disagreements can escalate when tension is high, it is not worth it!