My assumption: You are wishing to conduct a pinewood derby race using pinewood derby cars (typically for Cub Scouts). That means you want something that is simple to build and setup away from your home. Also, a benefit would be if race results could be tracked and displayed for the spectators.
If so, I would suggest you use a notebook computer along with the serial port RS-232 handshake lines hooked to photo sensors. You could then create a Visual Basic program to do the calculation. With some additional effort you could also keep track of the contestants and their individual times. If you know someone that has a projector that can be connected to the computer, then the race times and standing can be displayed for all spectators to see.
Newer notebook computers will require an USB to RS-232 converter.
Also USB based I/O devices are also available off the shelf.
Dale