Somewhere in genetic-land there's a copy of the gene that prevents the protein that holds the feathers into feather shape. Clear as mud....someone who knows more about silkie genetics will know more. Breeds of chickens can be "silkied" without being a silkie.
Cool they are silky feathered! Even with one pure Silkie parent the offspring do not have silky feathers. This is obviously a cool gene that has come out in your stock. You should definately work on developing it.
The gene for Silkie feathers is a recessive gene. Birds can carry the gene in a lineage for generations without it ever being expressed. Your parent birds are lovely mix breeds that probably had Silkie in their blood line several generations back. Both hen and rooster carry the recessive Silkie feather gene and as luch would have it the 2 offspring got the gene from each parent and it is expressed.