From the LEGO Community comes this optical theremin built from... you guessed it - LEGOs! The instrument features 2 optical sensors for controlling pitch and volume. Included on the website are the LEGO "schematics", a software patch to create the theremin, and a video demonstrating how it is played.
This looks like a fun and easy project to introduce junior scientists to the theremin. Hopefully you can work some magic on the software to improve the pitch range a bit!
Read: Light Theremin - NXT
Still got the LEGO theremin bug? Why not try this LEGO Tannerin while you're at it?