What little sweeties! They do look like mini pits lol. I suppose they could be missing some furries from fleas, mange etc, but ruling those out i expect they are just thin coated bc they're blue. Miley has a thin coat too, thin fur in some places and then just fuzz on her chest and nothing on the underside of her neck. If their thinness is more patchy I'd be inclined to think it's a medical thing.