One thing that keeps us from letting our kids explore art is ... well ... the art supplies! They are crazy colors and they do tend to get um, how do I say this - ALL OVER THE PLACE.
I've found that an old short sleeved t-shirt, preferably adult sized, makes a great smock. It's nice and long and if the shoulders are too wide, you can cinch it up in the back with a clothespin. Easy Breezy.