Possible that they dont know where to find it after being turned out? If the main waterer is inside the coop, try putting and ice cream bucket filled with water outside.... Maybe they are just having so much fun outside they forget to go in ????
If they have shade, I would estimate 26 would be needing about 1 gallon a day, maybe a little less at that age but real close to the gallon or so. How much are they drinking?