Are you able to feed your chickens a well-balanced, complete feed formulated for chickens? If so, they should not need anything else. If they have the opportunity to peck and scratch on some natural forage such as grass and forbs, and hunt for bugs, for part of each day, that should provide them all the extra "treats" and any additional protein they may need. Cat food is for cats, which are mammals. Their nutritional needs are very different from what a bird needs. I mean I know chickens are kind of omnivores and will eat anything, up to and including carrion, but that is not what is optimal for them. My opinion.