Kids nowadays are so showered with electronic goodies that I suspect a traditionally-lit birthday cake just wouldn't cut the mustard for some of them. Luckily, over at Instructables they've got a recipe for DIY electronic "candles" that actually lets you blow them out. Its flickering LEDs are accompanied by a thermistor warmed above room-temp by a nearby resistor, and accompanied by a microcontroller. When you puff hard on the thermistor, the circuit senses the temperature drop and switches off some LEDs. Brilliant, but sadly it seems you can't get extra wishes by blowing them all out at once. The demo video is undeniably funny though: a fake birthday part thrown by the builders.
All you need now to please an uber-geeky kid is to follow the instructions, box the device up somehow, and embed it in a cake. It's even a lower fire-risk than real candles... assuming your wiring doesn't go horrifically wrong. [Instructables]