Can you move the nest to a stand alone brooder/dog crate and set the legs in dishes of water to keep the ants from crawling up into it? Better yet move it into the house or garage where no fireants exist? If they must stay in the coop I think you are going to need a physical barrier (like setting the brooder legs in dishes of water/oil, or even vaseline around the legs of a stand alone brooder) to keep the ants from crossing over.
I have been using boiling water to get fireant mounds out of the coop, I tried molassis and it didn't work at all, but boiling water gets them to move the mound a couple of days later. Course in this case you need to protect against individual ants.
Personally I say get those babies and their momma into a safe area away from the coop until they are strong enough OR think of a way to prevent the ants from physically approaching the nest.