"It is a child's voice."
Translation:Het is de stem van een kind.
Why is it "de stem van een kind" rather than "een stem van het kind" or "een stem van een kind"? It's indefinite article for "voice" in English, not for "child". And in Dutch it suddenly becomes definite.
Good question. Since a typical child only has one voice, it would be strange to use an indefinite article. Compare to English: "a voice of a child" or "the voice of a child"? The latter is a lot more natural.
And because the English sentence doesn't specify which child we're talking about, we can't say "het kind" in Dutch.
By the way, the indefinite article in English refers to the child, not to the voice. Just like when you say "my child's voice", you mean "the voice of my child" and not "my voice of a child".