When I started, I think I was three, I remember having a lot of fun with singing, singing along, hand clapping/rhythm games, and just plain listening to music on tapes (we didn't have that many cds then). I've also seen people color-code the piano keys and have kids recognize color-coded music notes...very cute. I wasn't raised on the Suzuki method, but I've also heard it's pretty effective. I grew up on good ol' John Thompson, theory primer included. I'm sure you can get a hold of them at any store. Disney also sounds like a good idea...tunes they're familiar with.
Most of all, let them explore the sounds and the keys as much as possible. Rules and memorization will come later, if not naturally.
Also, try to determine their attention span early. My fascination allowed me to stay on the bench for hours from the start. My sister, however, could barely stay for 15 min. in her early years.
Hope this helps! Good luck!
