badBrick - Buzz Box, Complete Kit.

The Buzz Box is a complete kit. Its awesome for learning the basics to program and play with an Arduino UNO R3.
"Like US" https://www.facebook.com/badbrick

The "kit" is fully assembled and works as a basic motion detector unit. If you want to learn programming, or get your kids excited about what a computer and electronics can do, this is a great way to start. This kit requires it all. You need to get all the Arduino IDE set up, all the electronics connected. You need to write the code, compile it, upload it, and waaaaala. Magic happens.
http://www.badbrick.com/content/badbrick-buzz-box-complete-kit

For the most part its pretty simple if you know what you're doing.. For kids, its using a computer, having fun, and make something move. They learn a ton of stuff in the process without realizing it. They will want to know more.
If you want to make one yourself. Print the files. Our Kit includes:

1. Arduino UNO R3
2. Parallax PIR sensor, with extension wires.
3. Piezo buzzer, with extension wires.
4. LED
5. (2) 9 Volt batteries wired in parallel, with the Arduino Power plug adapter. This is the Power Pack for the Buzz Box.
6. A pre loaded basic motion sensing sketch and the downloadable source code.