Oooooh, I'd say that setting the cut-off at 14 would be a bit harsh. I'm 13 and I've been playing and singing with varying amounts of piano as a resident at folk clubs for the last year - well, ever since I got into folk music. It's all part of the experience, and I can definitely see how it's improved me as a player. I want a career in music - why shouldn't I be learning stage technique and delivery now?
Myself and a couple of slightly older friends often put some material together as a three - we've had everything from jazz piano, sax and fiddle to songs in three-part harmony. We're all experienced and capable musicians, so we've generally found audiences very appreciative, especially as we consitute the entire young folkie population of South Shields.
Encourage the young singer, I'd say - I wish that somebody had introduced me to folk and performing when I was 4 instead of sitting me on a piano stool and making me play strict classical. If the child genuinely wants to sing (and isn't too apalling) then you'll probably find the listeners very receptive, and able to make compensations for age. Be very careful about the father though - don't let him use his kid as a gateway. That's wrong.
Ruthie