I understand mud and all (i live in the country and own a ditch, mud is nothing) but as a parent I would be angry if I was picking my child up every day and they were filthy. I don't have time to clean up my child if they were dirty, make dinner, clean up and listen to my other children kwim.
sometimes you have to look at it as a parent, we dont' know what their plans are after they leave our house. And really, how hard is it to clean up a child befor they go home. Now if you have play time at the end of the day, like one of the above posters thats great, but if your going to do play outside all day then they should be somewhat clean to go home.