I hope you decide not to breed, many breeders who have spent literally thousands of dollars for their breeding dogs and have been studying breeding for years, have cut back on breeding. The economy is bad, and more and more backyard breeders are breeding and wanting to earn a little pocket money. This is really bad for any dog breed. A breeding dog should be as close to a perfect example of the breed as you can expect. You shouldn't have to ask anyone if it's a purebred, it should be obvious, but it also should have papers from a reputable registry. The Centennial Kennel Club will register mixes, and takes the breeder word for whatever it is, so it is not considered a reputable registry. Your dog should have health tests and you should know their background so that you can reduce the chances of passing on genetic problems. I hope you do more research before you consider breeding, here's an excellent article.
http://www.yorkietalk.com/forums/yor...-answered.html
By the way, your yorkies are very cute, but that isn't enough reason to breed them.