Grab your science supply catalogs and look for the chemistry demonstrations (I've gotten them from Flinn Scientific, Ward's Natural Science and Nasco) - they have kits that contain the chemicals (and often anything else you might need, minus the glassware) you need for each demonstration. Look for demonstrations that involve color changes or some other wow-factor. It's fun shopping, but if you're not careful, you could blow your whole camp budget on this one session - choose wisely and realistically.
I don't expect the students to learn a whole lot from this session (they're elementary students and there's no way they'd understand the chemistry), instead I use it as an opportunity to build their enthusiasm for science and just have a little fun (it is summer, after all!).