More than likely they are have Manx genes. Manx can have anything from a fuzzy stump to a full length tail, or anything in between. One of my cats has an almost imperceptably short tail, and the last few vertebra feel like they are knotted. This most likely means he has some Manx blood, especially since he has two other Manx characteristics - he is large and his back legs are longer than his front legs. These may not be noticeable in the kittens until they are a few months old at least.

I feel complelled to give you some advice you didn't ask for, though. I would bring one of the kitties over for several visits before you make a final commitment, and be very careful. Some dogs never overcome their cat predatory behavior. Even after I would make sure the dog and the cat are never left alone with access to one another.