For prereaders - learning piano involves so many new concepts, including the idea that we read notes on the page from left to right. You can start introducing this idea to toddlers by guiding their hand along the page in the hymn book under the notes or words as you sing each week in church. Sometimes this trick even helps them sit more quietly:) To go further, gently tap their finger to the rhythm of the song under the notes. They will pick up on a lot subconciously.
Before playing songs in the book have your child point to the finger #s starting on the left and moving across the page to the right while they listen to the corresponding CD song. Starfall has a short fun movie that also reviews this concept in the Learn to Read section.