Hi,
I am a composer who began teaching kids by accident. I really love it! It has been 8 years, and I am still trying to answer the question you pose. I have explored most mainstream curricula and I always return to developmental approaches (Kodaly and Orff teaching methods). Unfortunately there is not much support for those approaches in American Methods.
I really believe a curriculum should be ordered around harmonic complexity, not visual complexity. I have actually been using Bastien Piano for Adults recently with some kids, simply because it teaches them chords right away. It can be fun to get out colored pencils and help the kids explore sound.
As for something non-commercial,
http:/www.paintwithpiano.com. I have been developing colored sheet music for children (just to use in my own studio). I have found it really works for me. Even with really young beginners. Rhythm is so easy when you just speak it, and don't need to explain it. Kids are intuitive, and colored music seems to free the student from becoming frustrated with notation. They just play what they see, rather then trying to understand how to see what they have to play.
I would love it if you explored my website and gave me feedback as well. It is a work in progress, but I do have all eight levels mapped out. The first level is "done" and the next three are about 40% complete.
Books for beginners