Since you have a strong interest in visualizations of physics phenomena, and you're already teaching your nieces and nephews how to write Python, I'd like to suggest that you check out VPython, which is a series of 3D extensions to Python. In particular I think you'll be intrigued by these examples which visualize everything from wave superposition, to magnetic fields, to concepts from relativity. For immediate gratification, the author of that examples page also has Wiimote integration, so you can bridge interest that your relatives might have in video games into an interactive experience in your physics environment.

Good luck!