I find them unnerving and really quite freaky. The Sound of Music was on TV last night and starred Julie Andrews, who was one such musical mutant. She had a five octave range and debuted on the West End at about the age of 12, according to Wikipedia.
As I was browsing YouTube recently, I came across a performance by Bianca Ryan, who is reputed to be 11. It’s very, very odd to be watching someone that young sing and perform like that.