The most important thing in all of music are notes, since you know, they make up music and stuff. Learning the notes on a piano is not a super difficult task, which is why you should get your kid one. Even if you aren't ready to make that commitment we can use the piano structure to start teaching kids notes. One of the easiest ways for the littles is to combine this with something everyone loves, coloring.
The idea is simple: learn the basic layout of the repeating piano keys with the old color by numbers trick, slightly modified to color by notes. You will find that within an age appropriate week your little ones will be able to tell you which note is which, and by extension will know all of the white keys on a regular sized piano. We'll add the black key (sharps/flats) in a couple weeks.