Cat nip has no affect on kittens. They only get interested when they are around nine months old.
Pic.San Diego Humane Society
Dogs (and some cats) like cool treats just as kids do. Consider freezing Ultramix sweet potato or pumpkin puree dietary supplements in an ice tray and serve ice as a summer treat. Alternatively, turn the frozen treat into something special by freezing the mixture in small plastic cups and using a natural chewy treat as the stick. Insert the treat when the puree is partly frozen to get the stick to stand up properly. The treat should be easy to remove from the cup if you run the cup briefly under lukewarm water.