A very similar question was posted just over a year ago on Reddit's /r/math by a professor who had to speak to his daughter's class about what a math professor does. He was speaking to first graders so some would have been six, not five, but that's close enough.
Later, he posted a followup reporting how it went.
Summary: fractals, especially the Mandelbrot set. The kids went absolutely wild over it. They grasped the self-similarity. Parents told him their kids came home and would not stop talking about the Mandelbrot set.
