Most likely, the best book on the market for children learning chess. The really key point about it is that it doesn't just teach the moves but educates children on chess logic.
My children loved the story format whilst playing out the moves as we went along (probably a bit too impatient to do all the practice in-between chapters). As one reviewer noted, much of the material is available for free on Richard James' website but I would still recommend the book for the story, the "subversive" humour and cartoons. (Note - there is a fair amount of comic violence and emphasis on the war game element of chess - some adults may not be that keen on this but I think most kids love it).
I would particularly recommend for children between about 6 and 9 who have not played chess before.