hummm i think its the metal thing again, cannot see a problem with plastic ones, but my worry is nut could swallow some plastic she scrapes off the button etc also if wood, you dont know what its been treated with or chemicals absorbed when being washed
I'm making Lucy toys with all sorts of plastic buttons. She has a big square one my boss brought for her that she plays with at work. I agree with suebee, plastic is ok. Wood and metal are no-nos