Sometimes people I work with comment on the speed of my typing (40+ wpm). I’ve learnt touch typing many years ago and I always tell people that I learnt it through the need of completing essays. In reality, I’ve learnt and perfected my typing skills during my time in college, not writing essays but chatting on IRC!
When I was a small child, perhaps five or six, I played those computer games that were supposed to teach you how to type. Somehow, they worked. Since then, I’ve never had to struggle to type, never had to look at the keyboard, and consistently have typed speedily.
So, while your children are young, hook them on those kiddy typing games! It worked for me.