recording cycle speed and distance

[chil]

I have put this question in the sports section but hoped that some of you technology guys may be able to help.
I am running a sponsord event for young people where they ride bikes fixed to turbo trainers. I want a piece of kit that will record how fast and how far each bike is travelling and record the information on a central screen. Has anyone done anything like this and knows what I mean. Thanks for looking
Chil