LED light sensor to detect goal scored
I think you might want to have a look at the LEGO Mindstorms set since you have kids. You can actually build some pretty complicated stuff without knowing programming or electrical engineering. There is an ultrasonic sensor "able to detect an object and measure its proximity in inches or centimeters." Best of all it would be fun to learn and build with your kids.

If you're looking for something a little more advanced you could make a simple Arduino circuit to detect and send a signal to the PC. Take a look at this one. It sends an SMS signal to your phone, but you can disable that feature and implement a counter on the PC to display the score: http://techcrunch.com/2012/05/04/surprisingly-simple-arduino-motion-sensing-project-makes-for-some-weekend-fun/