Dehydration is pretty serious when they're that young, but you're on the right track with the sugar water. I would try using one of those packets of electrolytes (Sav A Chic I think they're called? Feed stores have them, or you can use Pedialyte from the infant section of a grocery store in a pinch). Other than that, just make sure the poor thing is warm and dry, and obviously segregated from the others until its strength comes back. Don't give up!!! I brought home a feed store chick that was in a similar situation (I complained about it to the manager since she was so obviously small and sickly, so they gave her to me for free) and I really thought she wouldn't make it. I gave her the electrolyte water every hour or so from the tip of a spoon (she wouldn't drink on her own either) and after two days she made a full recovery. She's now my best broody hen!
