First, love the comment on Gracie's behavior. I can totally picture her doing that. About Luna's dress, I agree that the date dress is great for her, but not formal enough. I think the dress you picked fits the setting way better.
For Gracie I would like something like this. Honestly, it is too much "woman" and mature for her, but I am just a sucker for an elegant dress that has that kind of open line below, showing the legs... And seriously, Gracie is gorgeous. Maybe it would work better on Isabel.
View attachment 1076118
Can see Cordia in something like this white and Isabel in black
View attachment 1076129
View attachment 1076134
Not perfect candidates but good enough for what I have in my head.