Lina Ng’s excellent Theory Made Easy for Little Children is a two-book series (both available from Waltons), specifically designed for children aged 4-7, presenting music theory using colourful illustrations and stickers for the exercises.
There are plenty of games to play which introduce the basics of music theory at a level small children can relate to.
This book follows on from level 1, taking children up to a stage where they can begin work for the ABRSM Grade 1 theory test. It incorporates all the fun and colour of the first book but works at a slightly more advanced level.