I don't know how helpful this is, but I make sure the kids have all the proper clothes (rubber boots, waterproof splash pants, etc) and just let them go to town in the mud and the puddles. Last year (before we put down new sod) our backyard was a big mud hole because half the grass was dead. So I let the kids dig and play with little construction trucks in the mud. There were some days I had to hose the kids hands and boots down before I would even consider letting them in the house.