This is probably not especially useful to anyone, but may be interesting to those curious about the inner workings of the Beetbox. The code for interfacing with the MPR121 is based on an Arduino example by Jim Lindblom.
I'm very open to people recreating the BeetBox as a learning exercise. The code and schematic in this repository are more or less exactly what I used in the project, though you may have to tweak things on your end. The main compontents you need are a Raspberry Pi and an MPR-121 capacative touch sensor board from SparkFun. I've also included the schematic for a simple amplifier circuit, but it is totally optional and you can easily use ready-made devices like computer speakers for amplification.
I used the stock distribution of Raspian from the Raspberry Pi foundation, though that was some time ago and I'm sure things have changed. The most important thing to make sure of in working with the MPR-121 is that I2C/SMBus connectivity is working properly. Adafruit has a helpful tutorial on Configuring the Pi for I2C.
Once your Pi is communicating with the board properly, you can clone this repository and run the script with something like sudo python beetbox.py
A list of links to purchase some recommended tools and parts is available on the project page.
Dealing with the Pi if you're not experienced with Linux can be a big task in itself. If you'd like to experiment with touch-based projects, but aren't quite ready for that level of complexity, there are a few easier ways to get started.
The MaKey MaKey works on a different principle (resistance rather than capacitance), but the results are very similar and it's extremely easy to get started with. I'd definitely recommend it for beginners and children.
For the initial prototype of the BeetBox, I used an Arduino Uno connected to an MPR-121 board (with code and instructions from Jim Lindblom) and plugged it into a laptop running a simple Processing sketch to trigger sounds. Another option would be to skip the sensor board alltogether and try the Arduino CapSense library.