The folks who run "ensemble", an electronic music workshop for children, have created several garments and accessories with simple sensors. There's a hat that senses its position, a bag that senses light and darkness, suspenders that sense when they're being stretched, suits that sense their distance from each other, umbrellas that sense pressure, and skirts that sense acceleration.
As the kids play dress up, their movements and actions modify sounds and allow them to physically explore the music they create.
- Thanks to Chad for this link!
Ah, but you forgot to mention STEIM's toys for the big kids. When STEIM was at NIME (New Interfaces for Musical Expression) they had two workshops: one for ensemble and another on hacking USB controllers. I didn't make it to either (out enjoying spring in Montreal), but both were well regarded.