My response to this is always, "Let them? You think I could stop it if I tried?"
No. I could not. Short of locking them inside the house and keeping them on leashes when they're outside. Neither option appeals to me, so they will go outside and they will get filthy. No question.
Though maybe the mud puddle is pushing it a little bit . . .
Charlie found a worm, which I admired appropriately. From a distance, because I'm not so into the invertebrates.
Literally down and dirty is the only way to go when you're under five and have access to mud.
The plane also got a mud bath. It was great for his pores.
Yeah, my kids are dirtier than most. It can't be helped. Sure makes for a lot of laundry, though.