Arduino source code
I've attached a wiring diagram created in Fritzing but the image may be too small. A larger version and source file can be found in the open source repository above.
Trick or treating, she was somewhat of a local celebrity and as you can see she got some bonus candy from people who wanted to try out the candy detector some more.
Full feature prototype on my desk, pre-packaging in costume:
Special thanks to Lee's Electronic in Vancouver! Awesome support and customer service.
Step 1: Concept Illustration
I sketched out a few concepts. The hardest part was coming up with a helmet design. I wanted a boxy, retro look. For a four year old, I didn't want anything heavy that her neck would have to support, so I opted out of separate piece helmets and went with one that was supported by the torso with a big "mouse hole" opening for the face. This also allowed her the best vision. A lot of cool helmet designs obscure vision a lot. I still ended up having to help her navigate stairs and walk past curbs and other tripping hazards because she couldn't see her feet.
I ended up not having time for features D and E.