How about leading them through an interesting problem, like a geometry IMO problem or, if that is asking too much, a Mathcounts problem? It could be set up on square tiles, the left most of which would contain the problem, with the following tiles showing the steps of the solution. It should be a problem that can be written such that viewers see a surprise toward the end, thereby possibly giving a glimpse into why mathematicians enjoy so much what they do. Although a Mathcounts problem would no doubt be accessible, a very beautiful IMO problem could be inspiring. Very likely, it would be entertaining for both children and their parents.
One might also include multiple solutions to a problem to dispel the notion that for each problem only one solution exists.
One can see examples of interesting presentations and ideas for problems at Rusczyk's Mathcounts channel at
http://www.youtube.com/user/mathcountsfdn
Finally, using
Using the same format from above, one could present a suitable Putnam problem and show its connection to research. This is discussed in Kedlaya, Poonen, and Vakil's book.
Finally, this response might be related to Kevin Lin's. and ein's.
