A structured system of games that gradually introduces the basics through a fun, colorful, and interactive approach would be awesome. I'm exploring at the moment and will report back.
I had a bit of a bomb lesson today, this one 4 year old is very rambunctious, attention wanders like mad. She got the previous lessons great, her mother sits with us and helps out, so that is a huge advantage. We were both surprised and very pleased with her sudden move to playing on the keys, groups of two black notes, but this week while what she got and showed me was wonderful, it was done in less than a few minutes and it was just too soon to do anything new. She resisted and was difficult the rest of the lesson, which was a bit embarrassing because I really couldn't do what I had planned.
I'm still new to teaching this age group, but I really do enjoy it. I need an approach that will really be fun and gradually introduce new concepts which get her up and active. She loves listening and playing along to songs, but its just the attention level is so quick that you really do need a wide range of places that you can go at any moment.