@SarahGen-- I disagree. I don't think that we are fully aware of the side effects of rubber mulch yet. This mulch might release chemicals or dangerous gasses into soil and air.
I don't know why rubber mulch is so popular for playgrounds. Playground chips made of wood are actually cheaper than rubber mulch and as long as it deep enough, it provides the needed cushion for children in case they fall. It's natural, environmentally friendly and absolutely hazard free. All of the municipal schools in our area use this type of playground mulch.