I think that you should let them be. Your dog will eventually decide to stop nursing and will wean the kittens. You certainly don't want to make her stop suddenly-think how painful that would be with all of that milk. That's when she would be most likely to get mastitis.
__________________  Shannon & Puff 
"All his life he tried to be a good person. Many times, however, he failed; for after all he was only human. He wasn't a dog." Charles M. Schultz |