My cat Gopa was the foster mother of two orphaned kittens...long after she gave birth and weaned her own kittens. When my vet asked me if Gopa was still lactating, I had to inform him that she weaned her babies and her milk had dried up. He told me that the more an animal suckles, the more milk she makes. He said that it even happens in animals who've never had babies.
I brought the two orphaned kittens to Gopa and, sure enough, a few hours later she was nursing. She didn't have the abundance of milk that she had when she gave birth, but she still had enough milk to suckle them and form a bond with them. I helped out by providing a bottle twice a day.
As long as the dog isn't bothered by the kitten nursing from her, then I would leave well enough alone. The dog will let the kitten know when enough is enough and will "wean" the kitten from her.
Here's another thought...having the kitten nurse from her might be a pleasurable feeling for you dog.
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“In a world where you can be anything, be yourself.”