Dairy based kefir is generally fine for dogs and cats. You can read a little bit more about kefir for dogs and why lactose isn’t an issue with kefir in the article I linked, but basically the beneficial bacteria also breaks down the sugar lactose turning it lactose free. If you are still concerned, there are also water based kefir grains (such as Water Kefir Grains from Cultures of Health on amazon) you can try as well.
One other thing you should be aware of if you purchase Kefir for your pets, make sure they are unflavored or plain and don’t contain any artificial sweeteners that could be dangerous. The article I linked to above has a simple easy recipe for making homemade kefir so you don’t have to worry about unwanted additives.
