Getting stains out of white fur?

Hey all, as with little babies they tend to poop on themselves. These little guys have white bellies, but they are kind of brownish yellow because of their accidents. I have been giving them daily baths to help try to get the staining out, but is there anything else I can do? They have to be adopted out in a couple of weeks, and I don’t think browned stained bellies are a desirable trait in potential adopted kitties.