I used AVKO (pdf file) for my children.
Its advantage was that the children did not feel pressured since no computer was waiting for a response or asking for a speedy response. Also the progression made sense.
After that, they did speed drills with some free online program.
C is making up his own typing program (or will be until I print the AVKO exercises out for him), and we're using an online typing test to check speed.
C's current speed: 10 wpm
OK, I just attempted to take the test and was, predictably, interrupted by household stuff just before the end. Still managed to score 100 wpm with 3 mistakes.
* Can't bring myself to speak of keyboarding.