First a dog would lick a cat butt probably for the same reasons they are eager to eat out of a litter box. As I've been told a cats food tends to be much sweeter than a dogs. That scent a dog can pick up even after a cat poops it back out. I would imagine your dog is just curious about the smell. And well unlike people he probably sees nothing gross about licking a cats booty. Secondly, I would imagine your kitten is okay with this treatment because its common treatment from a mother cat. Kittens cannot pee or poop without the simulation of the mother cat licking there booties. And mother cats do this to help their babies deficate and to do away with any scent her kittens leave behind. To protect them from the possiblity of predators. The kitten probably welcome the motherly attention. Afterall, cats paw at you went they purr because that's what they did as kittens to help stimulate a mother milk when nursing. Your pets are just curious babies at heart. I wouldn't incourage it because kittens almost always suffer from worms until they are dewormed by a vet and I wouldn't like to see your dog get worms from this behavior. Hope this helps.