I don't know if the children you're talking about are too young for this, but The New Cool by Neal Bascomb is always a great read. I've also heard of FTC Robotics: Tips, Tricks, Strategies, and Secrets, but never have read it myself.
Aim High:Behind the Design and Rack 'N' Roll:Behind the Design are also good books if you can get your hands on them, but they'd probably get boring for middle schoolers.