My son asked me to teach him typing today. I remember I used to play some computer games for typing when I started learning computer back in 1996. Game is a great way for kids to practice, so I searched for typing games. In the end I found BBC’s Kidztype very good.
It has 12 stages, from easy to difficult. We obviously started from Stage 1, where only the keys in the home row (asdfjkl;) are practiced. My son loves the game and keep playing it for ~1 hour.