Can you recommend a book to learn to teach math to a child?

Math From Three To Seven by Alexander Zvonkin is an excellent book that chronicles the author's experiences running a math circle for his children and their friends from the ages 3 to 7. It does not purport to be an instruction manual, but there are many great ideas and all activities are described in detail. Here's a preview.

And just to give some credence to his lessons' effect: his son Dimitri is also a mathematician!