I had never heard of Greenies until I stumbled into this forum.
However, personally I believe (and my vet agrees) that no chew bone or dental treat will clean a dogs teath as well as daily brushing. So if you are concerned about your dog's breath or dental health...I would start brushing. I suggest you use the finger type of brush (it makes it a whole lot easier in small mouths). The brand of toothpaste I use is PetDental. It costs about $6 and can be purchased at PetCo (or probably any other petstore). This has improved Sydney's breath 100% & SHE LOVES IT. The toothpaste tastes like butterscotch and she gets so excited she starts hopping around as soon as she sees the tube.
This form of teeth cleaning/breath improvement is 100% safe and effective...so why monkey around with a product that might harm your baby? Besides we all know that Yorkies are prone to tooth decay and stopping that will prolong the life of your pet.