New and in need of help!

I'm brand spankin'new here, though I've been using these forums for help for a while now. Just more...off the grid kinda thing.

I'm finding myself in quite a predicament. I purchased four pullets and three ducklings, and everyone seemed to be doing dandy until about a week ago. One of my ducklings began laying down frequently. Like...waddle a few steps, then plop, down for the count until she could get up. Then rinse and repeat. It got quickly worse (as in, within a day) to where she refused to get up at all. So I seperated her, because the other two were growing, and she kind of just wasn't. And I noticed they just walked all over her. She stopped eating and drinking, and one eye got all gunked up before I took her to her own private quarters.

After carefully cleaning her eye repeatedly for two/three days, the gunk cleared up. But then I noticed she was't moving her legs. At all. They just flopped and dangled like wet noodles. Since then she's started moving them again (a bit), but her feet and ankles are stiff as boards. Her toes are kind of curled, and she doesn't walk...just kind of shuffles about on her tum when she does move.

Their food is medicated, and water has soluable vitamin tabs put in it.She gets fed a gruel several times a day (in addition to crumble and vitamin water) with brewer's yeast and sugar, even more crushed crumble and cream of wheat. She has an apetite and life in her, but I'd like to know if anyone can help me out. I've never had a duckling with this kind of health issue before. I'd really rather not lose her! Any help is much appreciated.

Please and thank you in advance. <3