This dog has made its goals very clear, you will have to go through it before coming anywhere near the cats. It will lunge at you with furious anger and swipe those sharp jaws at you. They make for extremely efficient hunters and once they fall in love with someone, they will guard it with all their life. Dogs don't know of fear and cowardice, they just don't have the instinct for it.
They survived through the ages purely because of their killer instincts and their clever use of strength, intelligence and those deadly jaws. Those two cats have placed their complete trust at the dog which has probably rescued them from various hostile situations on more than one occasion. But what if this dog's killer instinct kicks and it ends up swallowing one of those kittens?