They call it bruxism, and it's harmless. It's generally something that a child does as their teeth come in, either to soothe teething pain, or just try out these new things in their mouth. It doesn't hurt to call the dentist, but usually they aren't concerned about it at this age.
My kids both did/do it, my son is 15 mos and still doing it. He's not wearing down his teeth, and it bothers me more than anyone, so I just let it go, as best I can.