I have heard that detergents can help set the stain so you definitely don't want to try that, unless it's a last option!
Good luck! It sounds like you are a rescuer. Good for you! That is such a hard job and yet so rewarding. I've rescued hedgehogs, though I currently only have one little guy left and he wasn't a rescue. My name and number are with the pound so if they ever get one turned in to them they call me. Normally if that happens, the little guys are in bad shape and need a lot of love and care. I can't imagine baby kittens are much different. I helped bottle feed a baby kitten whose mother had been killed (car ran over her) when I was younger so I have some respect for what you are doing. It's hard when they leave but knowing they are in a forever home is a wonderful feeling and you should be very proud of what you've accomplished!