This was interesting reading. I went to a Chroisters Guild convention once and heard about the Mable Boyton or Boyter method for teaching children to sing on pitch. It worked for a young lad in our junior choir at church and here is how it was done. You put your temple against the student's temple facing each other and close off the outside ear of the student so they can really listen to the sound they make. Then you hum a note and ask them to match the pitch. Another way is to ask them to sing a note and then you match it. It takes patience and a cooperative student.
I think that all children should be encouraged to sing from early in life. It's awful to tell a child to just mouth the words.