Simple Music is best.

The two note minor third melodies are the best place to start teaching toddlers. Simply play the recording, and show the two hand signs, for one minute. It is best if the mother or father do this, but the preschool teacher can do it as well.

http://www.angelfire.com/mac/mcclain/track3Teach.html

Later, the same two hand signs can show that the minor third is 'movable', that is, transposable, by a perfect fourth. You don't explain it, you just play the recording and show it, just now and then, not for more than a few minutes at a time.

http://www.angelfire.com/mac/mcclain/kindergartner.html

Then you do some silly fun stuff (Major Triad three-note melodies that prepare them for reading music in first grade):

http://www.angelfire.com/mac/mcclain/kindergartenlessons.html