That's a good idea, GF! The thing would be to find a good top for that.

Another option is to find a big statement necklace (pearls?) to fill in the gap. Perhaps that would give the illusion of coverage?

That would be nice, Meredith, although I don't know about pearls for someone very young. A blingy necklace may work but we still want the coverage

How did this go?

Sorry I did not give follow-up before, it's been one thing after the other.
She did not like the look of a cami under the dress. I got her so many cami options to try on that our home looked like another Nordstrom's!
Her friend shortened the straps of the dress and she chose properly fitting underclothes and some gold colored sandals with a nice wedge heel. She wore that for the ceremony and pictures and after that she wore a nice light colored jacket over the dress at the party. I think she looked very pretty and was able to navigate things in a way that worked for everyone. Sorry that I cannot share a picture, she will not allow that.
Thanks everyone for all the great advice.