When I got my Fae and Ulalena they were pretty filthy too and had possible metal poisonning from the rusty cage they were in. They were eating kibble and craisins.
Within days I noticed that their appetites were good and their peep and poop smelt "better", not as pungent.
Fae died due to health complications and her low bone density, but Ulalena is getting better everyday. I've had her for a month and a half now.
Yesterday I saw her foraging for the first time. She is much more active and dextrous. She still doesn't stretch much, or spread her membrane, but she is getting better.
Now that she is in with my other three, her fur is nicer. It still has some cracks in it, but it is definitely much better than it was.
Getting your babies into a clean environment and giving them a good diet will help them to turn it around to. All they need is time to get better.
Good Luck.