I learned when I was eighteen, and I have a typing speed of 100 wpm.
The basic teaching technique is very simple.
You place your fingers on the middle set of letters.
Your left fingers (not the thumb) from A to F, and your right fingers from L to H.
From there, you place your fingers on all the other letters. But you mustn't look at the keyboard. So you can have any type of computer screen, which shows you where to place your fingers.
For an adult, it takes a couple of months to be fully trained. A child would probably need less time to memorize, but obviously their spelling is not great, so it's swings and roundabouts.
It is also very obsessive for a while. When you learn to touch type, you find yourself tapping out the letters whenever anyone speaks to you!!
I think six is a little bit young, for reasons of concentration and spelling. But it's certainly worth thinking about, and messing about with for a couple of years.