im not 100% sure how much pain they feel when they are only a few days old ( not sure how well developed there nervous system is yet) but it would be the longest time form of euthinization if you think about it, it takes like 3 hours to freeze ice cubes so its at least the same amount of time to freeze them solid. if you use the vaccum they would run out of air very fast . if you dont have co2 then the vaccum would be the next humane way lol. you could wack them in the skull to instant kill they dont feel that but its if your up to it