For my kindergarten class, I've tried a bunch of different things and have found great success with using smooth river rocks to teach the concept of hot and cold. It's probably too abtract a concept to teach the kids what this means without them getting their hands on it. I've used very warm rocks (that I had sitting in hot water) and rocks I put in the freezer so that they were COLD! Then we took turns touching each one. Just be careful you don't go overboard getting the rocks or pebbles hot. Using the same object (as opposed to using ice cubes for one and using something else that could be hot) I think helps to drive home the difference you're trying to explain. You could also talk to the kids about how they feel when they're hot or cold. Maybe have them draw themselves hot and draw themselves cold.