Also available: Book 2
This set of books is worthy of a mention not because they offer any universal panacea for starting the teaching of theory to youngsters but because they are very well produced and nice and colourful. There are games to play, stickers to use and colourings to do, and it seems to makes theory as fun as possible. They also have the distinct advantage of being cheap. The more advanced books are worth investigating as well - and they're cheap too!
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).