I usually like to start small, with reactions kids may have seen before and that are safe for them to play with. Mixtures of baking soda and vinegar are a classic, but try swapping out the vinegar with other acids (lemon juice, for example). And if you have a thermometer, watch for the temperature drop as the reaction takes place. You can also do the reaction in a soda bottle, topped with a balloon, to show that carbon dioxide is generated during the reaction.
Another fun reaction is yeast and peroxide. Personally, I go for the easy packet of yeast directly into a cup of 3% peroxide from the grocery store. It yields lots of foamy fun. You can also make a solution of dry yeast in warm water, so that the little guys are awake and active before mixing, to see if that affects the reaction. My friend Elizabeth McCarthy from South Mountain Coop, also does the reaction with frozen peroxide and dry yeast. Because the reaction is exothermic (it generates heat), the reaction itself melts the peroxide and drives the reaction further.
One more classic reaction is Mentos and Coke. You can do a demonstration of a full roll of mint Mentos into a 2-liter bottle of diet Coke, which has considerable "Wow" factor, of course. But it's a great opportunity for scientific exploration as well. Give the kids several types of soda and candy, and have them work through various combinations. (There is a great Mythbusters episode on this reaction, which I encourage you to watch.)
Slightly more dangerous, and requiring a brief safety warning, are dry ice experiments. The sublimation process that allows the frozen carbon dioxide to go directly to gas is both scientifically interesting and fun to watch. It's important to note to students that, though we call this substance ice, it is not water at all, but rather a completely different material, created through pressure and deep freeze. After letting students observe pelleted dry ice for a while, add warm water and create some fog. Then add a bit of dish detergent -- the resultant bubbles filled with fog are very exciting and perfectly safe to touch. Later the foam will freeze into "snowballs" which are also fun for play, especially at the height of summer.
Then there is my favorite reaction, the most dangerous one I personally do: thermite. A mixture of powdered metal and a metal oxide, when heated the combination undergoes an exothermic oxidation-reduction reaction that sends sparks flying. And it is awesome. This is the stuff used in welding and fireworks. After everything cools, you can retrieve the blob of iron left behind as a fond memory of your favorite explosions.