I have been intrigued by the idea of the soil-free Marathon Lite sod sold by dog. com for pups and small dogs. It seems to grow in a hydroponic mat.
http://www.dog.com/item/pet-a-potty-marathon-lite-sod/102231/
The one time I wanted to use it, they were out of it.
I think that you could order several, and the pups would be trained to real grass. No soil, so no dirt, no worries of heavy metals, unless there is more to hydroponic style grass than I know. I would tend to want to rinse it and let it dry before first use.