Salt water boils at a higher temp than plain tap water. A simple project you could do is microwave distilled water to a high temp. When you add an impurity to it, such as a sugar cube it will come to a boil immediately. Distilled water, boils at an even higher temp, but when an impurity is added, it lowers the boiling very quickly. Make sure the kids are at a safe distance, the water may splash a bit. Have the experiment behind a splash guard of some sort, and the dropping of the sugar cube controlled from a distance.