Most 6 year olds can sing a comfortable Major sixth, some a good octave, and some beyond. It depends on their training, etc. I like to stay from B flat (below mid. C) to C above mid. C as a MAXIMUM, with C - A as the most comfortable range. Encourage them to use a "singing" rather than a "speaking" range, and they'll have no trouble learning to use their head voices. Watch very chromatic songs...half steps are a killer. HTH - Jan