I just got this book:
It is available at http://www.walmart.com/ip/4300800 for $2.82 with free shipping for site-to-store. (It ended up being $2.99 after tax for me.)
I believe that an adult without any piano experience could figure this out and teach it to their children if they invest some time too.
With Luna, age 6, I am teaching correct form and giving her the whole piano lesson experience. With Katie, age 3, I am just demonstrating the song and letting her pick it out with one finger. I think Katie will learn the tempo by ear and she will be happy she is playing the same songs her sister can play.
I hope to post some videos of the girls playing soon.
If you decide to try this with your children and have any questions please ask.