3 week old kitten

Momma cat has a big area that's boxed in on the front so that only she can get in and out not the kitten now she keeps moving him out where the dog and get him she stays with them most the time but the dog already got him once how can I make her stop moving him

You can't stop her from moving her kitten, so put the mom and her kittens in a separate room from the dog. Since the dog is attacking the kitten, they should not be in the same room. Put mama cat's food, water, and litter in this room with her and her kittens (like a bedroom), and do not allow the dog access!