For the sake of an argument, let's say Joe Blow is a singer, and his hair is curly. Does that mean it would be accurate to call him a curly-haired singer?
Not unless the curliness of his hair affects his singing--and is in fact the most important aspect of his singing.
Otherwise you'd call him a reggae singer (if he is), or a baritone (if he is), or an Auto-Tune singer (if he always needs it)--whatever is most helpful in characterizing him.
Ditto Jackie Evancho. She is a child, but by the standard I just proposed you wouldn't call her a "child singer."
After all, she doesn't sound anything like any other child singer now nor or at any past time in the history of recorded music. I've listened to Julie Andrews at a similar age. Ditto Beverly Sills. They're good at what they do (coloratura soprano), but they still sound like children--just children with agile voices and a wide range. The young Judy Garland is closer, but she was an alto, so her sound and her music selections were generally quite different.
You could call Jackie a soprano, a classical crossover singer, or maybe the singer with the richest tone in the business--which is true regardless of age.
But calling her a child singer implies that she's competing with other child singers.
She isn't. She's competing in her field with its leading lights today--Josh Groban, Andrea Bocelli, Hayley Westenra, Katherine Jenkins and the like.
And in the broader market she's competing head on with the best-selling pop singers in the English-speaking world--with Lady Gaga, Adele, Eminem...folks like that. Look at her album sales here and in the US (where she's been known for a few months longer), and those singers are her ballpark.
Jackie Evancho is a bestselling classical crossover soprano who regularly makes large number of people in her audiences get weepy. That's who she is.
