If this happens to you, about all you can do is shrug your shoulders and set to work making yet another paper mache volcano to be filled with baking powder, into which your own little genetic experiment pours some vinegar at the appropriate moment. However, we will not concern ourselves here with the last minute one off science demonstration, but rather a research project to be documented, written up, and mounted on a cardboard display board for judging in a science fair.
Crystal radios make for very good science fair projects.
A lot of the judges, particularly in the physical science areas, are guys like me, and we got our start with these little sets, so you get some added appreciation along with the nostalgia bonus. Little or no competition, particularly at the earlier grade levels - electronics projects are rare. Once the project is underway, which may require some assistance getting parts together and an antenna up, not much parental help is needed. Once the set is up and working, the kid can pretty much take care of the rest. The same equipment can be used as the basis for future science fairs.