PETER: (baldly) You see, Wendy, when the first baby laughed for the first time, the laugh broke into a thousand pieces and they all went skipping about, and that was the beginning of fairies. And now when every new baby is born its first laugh becomes a fairy. So there ought to be one fairy for every boy or girl.

WENDY (breathlessly). Ought to be? Isn’t there?

PETER. Oh no. Children know such a lot now. Soon they don’t believe in fairies, and every time a child says ‘I don’t believe in fairies’ there is a fairy somewhere that falls down dead. (He skips about heartlessly.)

J. M. Barrie (1860-1937) Scottish novelist and dramatist [James Matthew Barrie]

Peter Pan, Act 1 (1904, pub. 1928)
    (Source)

In Barrie's novelization, Peter and Wendy, ch. 3 "Come Away, Come Away!" (1911), this is rendered:

“You see, Wendy, when the first baby laughed for the first time, its laugh broke into a thousand pieces, and they all went skipping about, and that was the beginning of fairies.”
Tedious talk this, but being a stay-at-home she liked it.
“And so,” he went on good-naturedly, “there ought to be one fairy for every boy and girl.”
“Ought to be? Isn’t there?”
“No. You see children know such a lot now, they soon don’t believe in fairies, and every time a child says, ‘I don’t believe in fairies,’ there is a fairy somewhere that falls down dead.”