This is a tough one. I've had dogs and cats all of my life. Usually this kind of problem, for me, has been between two cats. One of the very first times I posted on Mudcat was about our dog Merlee who, when we first got him, when after my Siamese male and had him in his jaws before I could get to him. There is an old thread about it. The dog learned that when I said, "No kitty!" he'd better be good. I put a Velcro muzzle on him for a few days after that happened and told him to go to bed which was a kennel. I never had any problems after that.
That's not really something I would recommend for a cat of course and it sounds as though you've done a lot already. I would reiterate getting the cats a climbing tree or shelves or anything like that that they can get up on and looked down at the dog which cats love to do.
The other thing is most vets I've been to and doctors Foster and Smith online all recommend one litter box per cat plus an extra box. As an owner of, at one time eight cats, I can tell you that seems to be very important even though I never had nine litter boxes!
I don't believe in slapping or anything like that. If a cat is angry it is probably afraid; not the best of times to try to pick one up, but I don't think calming is any kind of reward for bad behavior.
If your niece wears those pants only while she's in bed, perhaps they don't have as much dog scent on them? I would see his coming in to sleep on her bed near the dog as his way of being curious and feeling safe about checking things out.
Eight weeks does seem like a long time but, cats don't like change and this is been a major one. In my experience with cats, especially, sometimes it just comes down to a truce of sorts with small skirmishes now and then. I think the dogs I've always had new from the moment they came in the house, it was a "cat house" and they would toe the line.
I wish you and your niece and the critters all the best and hope that it will work out.