OP here again. Tonight was the princess/stepsister dinner, and yes, my DDs both wore their nightgowns/gowns. They looked adorable, and didn't want to change out of them after the meal. Several castmembers told them how pretty they looked, and they felt special, I think. Glad we went for it! We saw many girls there who had been BBB'd, and they did not look very comfortable, and the dresses did not look markedly different, either.
The easy part? Not having to change them when we got home into PJs! Tucked 'em in in their royal gowns. I'll try to post pics tomorrow.