Help! Pasty butt
Are they picking at her bum?
Is it red?
After you soak/wash - don't pull off the poo try to dissolve it - dry her well then apply a little Bluekote or Pick No More, this will hide the redness. Once that dries you can put a little oil on the downy to help keep the poo from sticking.
What is the temp in your brooder and what type of foods/treats are you giving?
Any pictures of the new babies and your brooder setup?