Beginners Learn Piano - Materials for Children - Alfred's Prep Course
This is a hugely effective series especially when used with all materials at once (per level). Children find the books engrossing and enjoyable and learn fingers, then keys, then notes, then combinations and patters of notes, articulation etc. There is a system behind the method. The notes are quite considerable in size and the books have an abundance of illustrations. This particular series is aimed at 5-6-year-old children but can also be used with 3-4-year-olds with extra materials.
These books are currently very popular on the market and available for purchase in many different countries. They are not available to download for free.