straight after the first time I had listen this piece I tought the singer are:
"Piccolo Coro Mariele Ventre Dell'Antoniano di Bologna"
but, I'm not sure about.
In Italy the are some children choir that sing in this way. But, the most famous is this.

Quem canta esta musica? Onde a encontrou? Belissima!

very, very good, it is a very pleasure for the soul. thanks a lots!