Your babies are adorable!!!
As for the white on the chest, that is perfectly normal, it indicates that the puppy will have a nice thick coat when full grown and it will turn from white to tan once adult hood is reached.
I don't know why anyone would say such mean things about your pups and that they are not purebred. I have seen several litters of yorkie puppies and each pup in the litter looks different. They look 100% purebred to me and for someone to say they are not purebreds and are mixed just because they have white on their chest or because the two pups look different from each other, then that person really does not know much about yorkies!!!