It seems like "where is a boy?" has to be the correct translation, but that wouldn't make sense. I wrote "Where is the boy?", and it was accepted, but I don't understand why just "chłopiec", by itself, can correctly translate to THE boy.
The phrases on DL are taken out of any context... "Where is the boy" would be correct, if in previous phrase it was specified what a boy we are talking about, so the phrase in itself should also be correct.