Sensing sensors

Accelerometers may be something to look into. The are available in different sizes and varieties.
A professor at the University of Alberta developed a system using custom accelerometers that allowed measurement and logging of speed, distance traveled and position tracking that was mounted in a shoe. The rights were then sold to Nike.
You can do a search for accelerometers at any electronics supplier, such as Jameco or DigiKey.

I really appreciate the suggestion. Thank you.

One trade show I attended demonstrated a real-time system where lasers(I think) were set up at four corners of a room and a model wearing a set of passive tags on their clothing. As the model moved their body a display screen on the wall created a virtual person on the screen in 3-D posing and moving in the same exact motions as the human model did.

That being said, you haven't given us the slightest bit of information as to what you are trying to acomplish, so I have zero idea what information to offer you as a solution. The concept listed above is a stab in the dark and an effort to get you to consider that the sensors do not necessaily need to be attached to the body.

Accelerometers are a good bet, particularly the small three-axis units because you'll get all three components simultaneously. There are also six-axis units that measure rotational acceleration as well as linear, and the better ones (with very small dc offset) will let you integrate out to angular and linear velocities and positions.

If you can put an "exoskeleton"-like brace on the subjects (doesn't sound like it though for kids) you can go with encoders to measure angular joint position in real time.

If you are interested in stride and gait dynamics you might want to take a look at Tekscan's tactile imaging technology that can be put into shoes. It can show you dynamic pressure/force distributions on the sole of the feet with pretty high resolution.

Finally, I know there are systems out there that use multiple video cameras and stereography algorithms to monitor the positions of retroreflectors placed at key positions on the body. These are used extensively in the gaming industry to develop natural-looking body motion models, and in the movie industry for the same thing (think Gollum for example).

Most of this stuff, namely single-, three- and six-axis accelerometers and tactile imaging sensors, can be found over on the GlobalSpec site. Good luck - sounds like an interesting project.

What about using RFID tags/transponders? They can be mounted on an eklastic band for reusuablity/transferability as well as differentiation between limbs.

Look into the animation field where they use motion capture the map human motion.