First, your yorkies are not standard and should not be bred. Do you know their health line, their parents, their grandparents, done any health testing, etc? A breeder can chime in and explain more but breeding is NOT easy and should be done by everyone especially not by bad examples of the breed. That is not to say your dogs aren't beautiful. My boy is 15lbs and I prefer him this way but I would never breed him, especially if you don't even know 100% if your dog is a full Yorkie (like me, I think Jackson may be a Silky, but unsure).
Silky Terriers and Yorkshire Terriers are two different breeds.
Here is a good example in differences:
Home of the Gorgeous Silky Terrier!! - Compare the silky to....