So, she's been weak since she hatched? Is their feed medicated? If so, might want to switch to non-medicated so that you can give her vitamin supplements that include B Vitamins (especially good for the legs). Is the mother still allowing them to snuggle up to her? If not, do you have a source of heat for the chicks? Even a plain, non-teflon-coated light bulb would do for some added heat. Have you checked her vent to make sure it's not plugged up? Clogged plumbing in a chick can bring them down really fast. How does her crop feel when she goes to bed, and again how does it feel in the morning before being offered food or water? It should be full at night, and empty by morning.
Do the birds have access to grit to help them grind their food? If so, you may consider making a mush with some of their feed and add mashed sardines and/or hard boiled eggs with it. Good balance with their regular feed, extra protein & calories with the sardines (and usually irresistible to chickens), extra protein plus all of the amino acids that chickens need to support good health in the eggs (the yolk is the best part).
Sometimes a chick just fails to thrive regardless of efforts to keep them alive. I'll be keeping your feathered family in my prayers.
