"Which children are yours?" should also be valid, at a guess.
Which of them is your boy?
I think it is possible to say too
No, because ¨niños¨ is plural and ¨boy¨ is not.
automated voice on this on is pretty bad
Since the final syllable ends in "s", why does "Cuáles" require an accent on the "a"?
Question words require accents
How are you? ¿Cómo estás?
Where is it? ¿Dónde está?
Which is it? ¿Cuál es?
and so on