My children started in this theory book when they were 4 and 5. It was so much fun for them that every day they were eager to do more. It is perfect for young children with the use of colour pictures, puzzles and games, and easy exercises some of which have stickers that you use to fill in the answer, others you fill in yourself. Mine were always especially excited when they got to another one of the sticker activities.
It takes children steadily through the basics of theory (note names and lengths, time signatures, etc.) in a very gradual way. There is enough repetition to ensure the child knows their stuff, but there are not pages one one thing, just an explanation of the point to be learned and a fun activity to practise using it. Not bad for the price either.