Oposums like to eat both plants and meat. They mostly eat small creatures, like cockroaches, flies, frogs, rats, spiders, and mice. That being said, something is obviously getting your kittens, and it would be a very good idea to put them somewhere they can remain safe while they grow up. Take the kittens and their mom inside or set them up in your garage, a shed, or a barn where predators can't reach them.