Wish one of the kiddos had done this at the Science Conference. The closest one I saw that is related to this was the velocity powered laser pointer.
Andrew Walbran (I seriously recommend you guys check out his website and other projects as well) has built his homemade laser tag. What you need appears to be a toy gun, laser diode, a PIC16F628A microcontroller and some misc. components.
Check out full details here, on creating your own Laser Tag.