Do let your boys be boys.
Playing in the mud, driving bikes through a mud puddle, playing cars and trucks in it, and of course making mud pies is wonderful fun for messy toddler boys!
Yes, they are experiencing the sensaation of oozy, wet mud, but mostly they are simply enjoying playing.
Pop them outside in an old set of clothes, then strip them down in the laundry when they are done and pop them in a lovely warm bath.
It will be the highlight of their day!
