I finally figured out which one is Bridget and which one is Serena. Serena has a black rim around the beginning of her beak, Bridget does not. Also, now that they are getting their big girl feathers in, Bridget has a lot more white on her head and breast. Still have trouble when they are head down and facing away, but at least now I know what to look for. Still don't know why they are 2/3 the size of the other girls.
We have a number of them who look too much alike to tell apart, even as adults. We make sure they are banded differently, either with different colors, or one on left leg, one on right... that helps us keep track and when it comes to any needs for special attention it makes it even more important.