This is a response to a new early childhood music teacher who had a lot of questions and thinks starting early makes the most difference.
First off, just sing and chants tons and tons of repertoire. Find out what they really like and do a bunch of it. Play great great music. Use CDs of the Masters (classics, jazz, blues, all styles are great to expose young ones to—just have to find the right content). Dance to it and be expressive. GO crazy over the top in modeling your enjoyment of what you do. Don't talk much "about" anything. On the other hand, I do tell children when they're not using their singing voices, at age 3 or so. At 4 or 5, I tell them they are singing different tones than me if they're not in tune. I distinguish tones from rhythm by having them show me their singing voices for tones and their talking voices for rhythms. I do a wide mix of individual patterns—diatonic and arpeggiated patterns—with a mix of sounds and syllables. This is especially so in 2-3 year olds. I find that singing ooo-ooo, say on D to A above middle C in the key of D helps children use their singing voices. Then I use the syllable BUM when they're ready—very infrequently before 4 years old. You have to keep listening and learning from the children. They'll demonstrate for you what their needs are if you are challenging them appropriately. Sometimes I stretch too far, other times not far enough. That's why I like EC sooo much. Yes! I'm with you that this will make the most difference in the long run.