Where we live, there are two kinds of mud. City mud is rancid and foaming with toxins; cigarette ash, old cooking oil, spit, urine, decomposing food, leaking plastics, and other delights.
Needless to say, we don't let our kids anywhere near the stuff.
But when we go up the mountain to our buddy's organic farm, where the air smells of peach blossoms and mist, and even the pigs are clean enough to touch, we turn our kids loose in the "clean" mud.
Add to that some grilled hotdogs and a clevered watermelon, and a good day is had by all.